This pull request implements linter test helpers to reduce the verbosity
in the linter rule tests. The new `createLinterTest()` helper provides a
cleaner API with `expectError()`, `expectWarning()`,
`expectNoOffenses()`, and `assertOffenses()` functions.

Example:

```ts
import { SomeRule } from "../../src/rules/some-rule.js"
import { createLinterTest } from "../helpers/linter-test-helper.js"

const { expectNoOffenses, expectError, assertOffenses } = createLinterTest(SomeRule)

describe("SomeRule", () => {
  test("no offenses", () => {
    expectNoOffenses(`<div></div>`)
  })

  test("with offenses", () => {
    expectError("Error message.")
    expectWarning("Warning message.")

    assertOffenses(`<div></div>`)
  })
})
```

Resolves marcoroth#461