[py] Scope py_test_suite shared library to support files only

[AutomatedTester]

Summary

• py_test_suite() previously bundled all srcs (both test files and support files) into one shared py_library, and every generated pytest_test depended on that library
• Changing any single test file dirtied the whole library, causing all tests in the suite to be marked affected and rebuilt — roughly 300+ targets for a one-line change in one test file
• Now the shared library only contains non-test support files (helpers, utils.py, webserver.py, __init__.py, etc.); test files are no longer bundled into it
• Each pytest_test still depends on the support library for shared fixtures and helpers, so test execution is unchanged
• A change to alerts_tests.py now rebuilds only the alerts_tests-* targets (~6) instead of the full suite (~300+)

How it works

The fix separates srcs into two buckets using the existing _is_test() predicate:

support_srcs = [src for src in srcs if not _is_test(src)]

The shared py_library is only created when support files exist, and each pytest_test receives deps pointing to that support-only library (or the caller's deps directly if no support files exist).

Test plan

• bazel query 'labels(srcs, //py:test-chrome-test-lib)' returns only support files, no *_tests.py files
• bazel query 'kind(py_test, //py:*)' still generates all 699 individual test targets
• Existing Python unit and browser tests pass