Update test cases to work with multiprocessing start_method = "spawn"

[achimnol]

Currently the following 4 cases are failing on macOS since Python 3.8 as multiprocessing's default start method has changed "spawn":

• tests/test_server.py:414 test_server_user_main_custom_stop_signals
• tests/test_server.py:450 test_server_user_main_tuple
• tests/test_server.py:508 test_server_extra_proc
• tests/test_server.py:543 test_server_extra_proc_custom_stop_signal

To make them working, we need to allow passing extra arguments to the user-provided main functions and rewrite the tests to use module-defined inner main functions so that those could be picklable.

[achimnol]

Now it's resolved by using GitHub Actions and os.setpgrp().