TST,BUG: Us context from spawn to fork in (#22204)

Since Python 3.8, the default start method for multiprocessing has been changed from fork to spawn on macOS
The default start method is still fork on other Unix platforms[1], causing inconsistency on memory sharing model
It will cause a memory-sharing problem for the test test_large_zip on macOS as the memory sharing model between spawn and fork is different
The fix

Change the start method for this test back to fork under this testcase context
In this test case context, the bug that caused default start method changed to spawn for macOS will not be triggered
It is context limited, so this change will not affect default start method other than test_large_zip
All platforms have the same memory sharing model now
After the change, test_large_zip is passed on macOS
https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods

Closes gh-22203

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