[feature] Handle CONAN_RUN_TESTS together with self.should_test

[klimkin]

Conan already has condition to handle self.should_test. Expand the expression to cover CONAN_RUN_TESTS environment variable:

Current:

    def test(self, args=None, build_dir=None, target=None, output_on_failure=False):
        if not self._conanfile.should_test:
    return

Proposal:

    def test(self, args=None, build_dir=None, target=None, output_on_failure=False):
        if not self._conanfile.should_test and tools.get_env("CONAN_RUN_TESTS", True):
    return

CONAN_RUN_TESTS documentation.

• I've read the CONTRIBUTING guide.

[klimkin]

A better solution could be to take CONAN_RUN_TEST into account when setting conan_file.should_test in cmd_build method.

[memsharded]

Implemented in #8687, will be in Conan 1.35