fix(gazelle): Do not create invalid py_test rules in project generation mode

[amartani]

Since #1538, when using gazelle:python_generation_mode project, a py_test rule is created even when there are no test files in the project. fb673ee (first commit on the PR branch) reproduces this issue - it shows that a py_test rule is created even when there is no test entrypoint file (__test__.py) nor any test file in the entire project. Additionally, test rules created on project or package generation mode will always set main = "__test__.py", even when that file doesn't exist.

This PR fixes it by only generating a py_test rule if it can find some test file - either an explicit __test__.py, or any other file prefixed with test_ or suffixed with _test.py. It also changes the generated test rule to only add main = "__test__.py" if there is an actual __test__.py file. Note that, in the case where a __test__.py file doesn't exist, the generated py_test rule is likely invalid as-is due to the lack of main; this can be fixed by mapping to some other py_test implementation, and I believe the new behavior makes more sense than pointing main to a non-existing file.

It may be useful to review per-commit (all tests pass on each commit):

• First commit reproduces the issue;
• Second commit fixes the issue and the corresponding tests;
• Third commit adds additional test cases.