The following principles may help you write better automated tests. They are excerpts from the Angry Tests book. To get a more detailed explanation you may want to read the book.
[2.1] Keep every test shorter than a dozen lines.
[2.2] Assert only once per test.
[2.3] Assert in every test.
[2.4] Finish every test with an assertion.
[2.5] Don't use assertDoesNotThrow.
[2.6] Don't fail explicitly using fail.
[2.7] Use irregular input values, instead of "foo".
[2.8] Use different inputs in every test, don't reuse them.
[2.9] Always write failure messages.
[2.10] Don't share any data between tests; never use setUp and tearDown.
[2.11] Don't define constants (static literals) inside tests.
[2.12] Don't inline constants; assign them to variables first.
[2.13] Don't bypass object interfaces, especially not via Reflection.
[2.14] Name tests as full English sentences starting with a verb, e.g., buildsHtmlPage.
[2.15] Never write any comments inside or outside of test methods.
[2.16] Don't assert on behavior not promised by the object's contract.
[2.17] Assert on the most vulnerable pain points.
[2.18] Don't test getters, setters, and similar primitive functionality.
[2.19] Don't delete tests, disable them instead.
[2.20] Never test private or protected object methods.
[2.21] Close resources after use, such as files and sockets.
[2.22] Don't write feature code that is only used in tests.
[2.23] Assert at every step while getting to the end of a use case.
[2.24] Don't assert on side effects, like logs.
[2.25] Assert on all possible intermediate results.
[2.26] Don't test constructors — they are code-free anyway.
[2.27] Clean up before a test, not after it.
[2.28] Don't use mock frameworks, build fake objects instead.
[2.29] Don't forgive incorrect behavior, disable the test instead.
[2.30] Aim for one-statement tests.
[2.31] Use Hamcrest.
[2.32] Don't be discouraged from writing bad tests; they're better than nothing.
[2.33] Every time you change the code, add more tests.
[3.1] Classify tests as "fast" (50ms each) and "deep" (integrating everything).
[3.2] Make tests flaky and unstable, then expect bugs to be reported and fixed.
[3.3] Create custom matchers and reuse them in assert statements.
[3.4] Reproduce bugs with the minimum possible scaffolding.
[3.5] Tests must reproduce particular bugs, not successful usage scenarios.
[3.6] Don't assert on unimportant details, don't be pedantic without reason.
[3.7] Invent a DSL and write test stories using it.
[3.8] Use randomizers to generate test data.
[3.9] Don't help your tests; create the most inconvenient environment for them.
[3.10] Don't keep temporary files next to the source code; use a temporary directory.
[3.11] Don't test abstract classes.
[3.12] Don't let tests log anything; keep the log empty.
[3.13] Parameterize tests.
[3.14] Stop tests on timeout; don't let them run forever.
[3.15] Don't sleep for an arbitrary number of seconds; instead, wait for an event.
[3.16] When it's necessary to simulate a hang, sleep for a billion seconds.
[3.17] Let different tests test the same part of the feature code.
[3.18] Test whether your objects are thread-safe: in Java, in Ruby.
[3.19] Let tests retry when the behavior they are testing is flaky.
[3.20] Use tags to classify tests.
[3.21] Let your testing framework repeat some tests to increase the chance of hitting the bug.
[3.22] In tests for thread-safety, utilize all available CPUs.
[3.23] Run all tests with no Internet connection; they must pass.
[3.24] Don't assert on the details of errors.
[3.25] Keep the scope of try/catch as small as possible.
[3.26] Strictly one test per feature file: jtcop.
[3.27] Don't use verify() from a mock framework.
[3.28] Don't use PowerMock or similar frameworks.
[3.29] In tests, don't instantiate objects or call their methods with default arguments.
[3.30] Don't abbreviate; use curl --silent instead of curl -s.
[3.31] Don't let feature objects do the job of tests — verify inputs and state.
[3.32] Use decorating invariants to catch improper use of objects during testing.
[3.33] Run tests in parallel threads: threads.
[3.34] Every fast test must take less than 100 milliseconds.
[3.35] Don't mock the file system.
[3.36] Make the place with temporary files accessible after the end of the test suite: mktmp.
[3.37] Don't assert on the content of logs generated by feature code during tests.
[3.38] Use ephemeral TCP ports.
[3.39] Don't use inheritance to reuse test tools.
[3.40] Don't be scared of long test classes — they are OK.
[3.41] Don't fix feature code in response to a flaky test failure.
[3.42] Use maybeslow or a similar library to diagnose long-running tests.
[3.43] Kill long-running tests on timeout.
[3.44] Inline fixtures instead of keeping them in fixture files.
[3.45] Don't keep large fixtures in static files; let tests generate them.
[3.46] Create fixture objects that generate large fixtures at runtime.
[3.47] Keep reference fixtures as static files in the repository.
[3.48] Code duplication in tests is the last problem to fix.
[3.49] Extract test libraries.
[6.1] Test for grammar mistakes and typos: languagetool. typos-action.
[6.2] Employ Property Based Testing tools: jqwik, quickcheck.
[6.3] Use Fuzzing tools: jqf, oss-fuzz, syzkaller.
[6.4] Use Mutation Testing: pitest.
[6.5] Check files layout during the build.
[6.6] Use Load Tests: jmeter, locust.
[6.7] Use Performance Tests: jmh.
[6.8] Benchmark new vs. previous builds: jmh-benchmark-action.
[6.9] Use Linters: eslint, rust-clippy, checkstyle, pylint, ruff.
[6.10] Use Static Analysis at build time: clang-tidy, spotbugs, infer.
[6.11] Collect source code metrics and fail when thresholds are exceeded.
[6.12] Test SQL queries: pgtap.
[6.13] Detect slowest SQL queries: dexter.
[6.14] Use Sanitizers.
[6.15] Use Dynamic Analysis tools.
[6.16] Employ Clone Detection tools: simian.
[6.17] Use In-Browser Testing: selenium, playwright.
[6.18] Use Multi-Browser Testing: saucelabs.
[6.19] Use Cross-Browser Testing.
[6.20] Use GUI Testing.
[6.21] Use Cloud Code Analyzers: SonarQube, Snyk, Codacy.
[6.22] Create Live Tests that verify production-ready configurations.
[6.23] Use Health Checks after deployment.
[6.24] Generate tests at build time: randoop.
[6.25] Employ Security Tests: zaproxy.
[6.26] Do License Compliance Testing: ort, reuse-action.
[6.27] Detect redundant tests.
[6.28] Validate architecture at build time: archunit.