gh-108388: Split test_multiprocessing_spawn (#108396)

Split test_multiprocessing_fork, test_multiprocessing_forkserver and
test_multiprocessing_spawn into test packages. Each package is made
of 4 sub-tests: processes, threads, manager and misc. It allows
running more tests in parallel and so reduce the total test duration.

Lib/test/test_multiprocessing_forkserver/test_processes.py

@@ -0,0 +1,7 @@
+import unittest
+from test._test_multiprocessing import install_tests_in_module_dict
+
+install_tests_in_module_dict(globals(), 'forkserver', only_type="processes")
+
+if __name__ == '__main__':
+    unittest.main()