TST,BUG: Use fork context to fix MacOS savez test

[charris]

Backport of #22204.

Why the test failed

• 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

1. https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods

Closes gh-22203.