Deflake fork tests

[pcmoritz]

Why are these changes needed?

This fixes #36001

Related issue number

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  • This PR is not tested :(

[pcmoritz]

Actually after looking at this more, I realize we actually don't need the subscribers in the worker processes. So a better course of action that keeps fork working in macOS would be:

1. Remove the import thread
2. Instantiate the following variables only on the driver: worker.gcs_error_subscriber, worker.gcs_log_subscriber, and get rid of worker.gcs_function_key_subscriber which is not needed any more after we get rid of the import thread

[pcmoritz]

(^ I made sure the above deflakes the test on a local branch)