fix(py_test): Support target names with a test_ prefix

[nahueespinosa]

Adds a test_main argument to py_pytest_main and propagates it from py_test. Uses a leading "." in the filename to suppress pytest collection.

In the current implementation, when pytest_main = True, the generated test entry file is named after the target with a .pytest_main.py extension. If the target name starts with test_, pytest may mistakenly collect the file as a test module, leading to confusing import errors.

==================================== ERRORS ====================================
_________ ERROR collecting examples/pytest/test_prefix.pytest_main.py __________
ImportError while importing test module '/home/nahue/.cache/bazel/_bazel_nahue/eb153f5901d0d2e97fac7a89d3cffa65/sandbox/linux-sandbox/402/execroot/aspect_rules_py/bazel-out/k8-fastbuild/bin/examples/pytest/test_prefix.runfiles/aspect_rules_py/examples/pytest/test_prefix.pytest_main.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../../../../../../../../../../../external/python_toolchain_x86_64-unknown-linux-gnu/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
E   ModuleNotFoundError: No module named 'test_prefix'

Alternatively, if this naming convention is decided to not be supported, we could maybe warn the user in a more friendly way when py_test is used with a name that may trigger this issue.

---

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

fix(py_test): Target names with a test_ prefix do not generate an import error when specifying pytest_main = True.

Test plan

• New test cases added

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Test

4 test targets passed

Targets

//docs:update_2_test [k8-fastbuild]                           39ms
//examples/pytest:pytest_test [k8-fastbuild]                  2s
//examples/pytest:sharded/test [k8-fastbuild]                 3s
//examples/pytest:test_prefix [k8-fastbuild]                  1s

Total test execution time was 6s. 38 tests (90.5%) were fully cached saving 1m 5s.