Disable gRPC fork support in PortableRunnerTestWithSubprocesses

[shunping]

Applying the fix in #38566 to its parent class.

Redundant setup and teardown in the multi-worker subclass are removed to inherit them cleanly.

Test failure example:

• https://github.com/apache/beam/actions/runs/26552726280/job/78218062486
• https://github.com/apache/beam/actions/runs/26615956510/job/78431613222

________ PortableRunnerTestWithSubprocesses.test_custom_merging_window _________
[gw5] darwin -- Python 3.12.10 /Users/runner/work/beam/beam/sdks/python/target/.tox/py312-macos/bin/python

self = <apache_beam.runners.portability.portable_runner_test.PortableRunnerTestWithSubprocesses testMethod=test_custom_merging_window>

    def test_custom_merging_window(self):
>     with self.create_pipeline() as p:
           ^^^^^^^^^^^^^^^^^^^^^^

target/.tox/py312-macos/lib/python3.12/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner_test.py:1284: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
target/.tox/py312-macos/lib/python3.12/site-packages/apache_beam/pipeline.py:654: in __exit__
    self.result.wait_until_finish()
target/.tox/py312-macos/lib/python3.12/site-packages/apache_beam/runners/portability/portable_runner.py:572: in wait_until_finish
    raise self._runtime_exception
target/.tox/py312-macos/lib/python3.12/site-packages/apache_beam/runners/portability/portable_runner.py:581: in _observe_state
    for state_response in self._state_stream:
                          ^^^^^^^^^^^^^^^^^^
target/.tox/py312-macos/lib/python3.12/site-packages/grpc/_channel.py:538: in __next__
    return self._next()
           ^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <_MultiThreadedRendezvous of RPC that terminated with:
	status = StatusCode.DEADLINE_EXCEEDED
	details = "Stream removed (Deadline Exceeded)"
	debug_error_string = "DEADLINE_EXCEEDED:Stream removed (Deadline Exceeded)"
>

 ...

target/.tox/py312-macos/lib/python3.12/site-packages/grpc/_channel.py:956: _MultiThreadedRendezvous
----------------------------- Captured stderr call -----------------------------
INFO:apache_beam.runners.portability.abstract_job_service:Running job 'job-b448bab8-ee74-4885-804c-2fbb1c2fc353'
INFO:apache_beam.runners.portability.fn_api_runner.worker_handlers:starting control server on port 51713
INFO:apache_beam.runners.portability.fn_api_runner.worker_handlers:starting data server on port 51714
INFO:apache_beam.runners.portability.fn_api_runner.worker_handlers:starting state server on port 51715
INFO:apache_beam.runners.portability.fn_api_runner.worker_handlers:starting logging server on port 51716
I0528 03:33:05.777477  106607 fork_posix.cc:71] Other threads are currently calling into gRPC, skipping fork() handlers

[shunping]

r: @derrickaw

[gemini-code-assist]

Summary of Changes

Hello, 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 intermittent test failures caused by gRPC fork support issues during subprocess execution. By centralizing the environment configuration in the base class and cleaning up redundant overrides in subclasses, the test suite becomes more stable and maintainable.

Highlights

• gRPC Fork Support: Disabled gRPC fork support by setting the GRPC_ENABLE_FORK_SUPPORT environment variable to 'false' in the PortableRunnerTestWithSubprocesses class to prevent test failures.
• Code Cleanup: Removed redundant setUpClass and tearDownClass methods from the PortableRunnerTestWithSubprocessesAndMultiWorkers subclass, allowing it to inherit the configuration cleanly from its parent.

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[gemini-code-assist]

Code Review

This pull request refactors portable_runner_test.py by moving the setUpClass and tearDownClass methods, which manage the GRPC_ENABLE_FORK_SUPPORT environment variable, from PortableRunnerTestWithSubprocessesAndMultiWorkers to its parent class PortableRunnerTestWithSubprocesses to avoid duplication. The reviewer recommends using cls.addClassCleanup within setUpClass instead of a separate tearDownClass to ensure environment variables are cleaned up properly even if super().setUpClass() fails.

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