Automatically handle CONAN_RUN_TESTS environment variable

[klimkin]

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

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{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.}

[memsharded]

Besides the detail of the new helper, this looks good and could be merged for next release.

[memsharded]

Merged for 1.35. If you could contribute the docs as well, that would be great (I think a couple of lines in the build helpers docs and in the CONAN_RUN_TEST reference would be enough)