tests: make py_cc_toolchain test of headers/includes work with Bazel 9

[rickeylev]

Bazel 9 has a small change in cc_library behavior: the includes attribute populates
the .includes instead of .system_includes. This is OK for rules_python, since both
result in the includes being added as system include paths.

To fix, change the test to look at both; as long as the include paths are in one,
then its OK.

Fixes #3239

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a compatibility issue with Bazel 9 by updating the py_cc_toolchain header tests. Specifically, it adapts to a change in how cc_library propagates include paths, ensuring that the tests correctly identify necessary headers whether they are exposed via .includes or .system_includes. The changes also involve minor refactoring of test support files for improved clarity and maintainability, ultimately making the test suite more resilient to future Bazel updates.

Highlights

• Bazel 9 Compatibility: Updated py_cc_toolchain tests to correctly handle a change in Bazel 9's cc_library behavior, where includes now populates .includes instead of .system_includes.
• Flexible Include Path Checking: Modified the test logic to check both .includes and .system_includes for the presence of required header paths, ensuring robustness across different Bazel versions.
• Test Infrastructure Refinement: Renamed test-related artifacts (e.g., fake_header.h to py_header.h, fake_include to py_include) and introduced a filegroup for better organization of header files.
• CcInfo Exposure: Enhanced cc_info_subject.bzl to directly expose the underlying CcInfo object within test subjects, providing more direct access for assertions.

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[gemini-code-assist]

Code Review

This pull request updates the py_cc_toolchain tests to be compatible with a change in Bazel 9 regarding how cc_library's includes attribute is handled. The changes correctly adapt the test to check both .includes and .system_includes fields. The test assets are also updated and renamed for clarity. My feedback includes removing a leftover debug print statement and a suggestion to improve the test subject abstraction to avoid breaking its encapsulation, which would improve long-term maintainability.