Automatically handle CONAN_RUN_TESTS environment variable - #8687

Helps to avoid boilerplate in conanfiles:

if tools.get_env("CONAN_RUN_TESTS", True):
    self._cmake.test()

Implements #7965

Changelog: Feature: Automatically handle CONAN_RUN_TESTS to avoid extra boilerplate.
Docs: conan-io/docs#2056

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^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}