gh-103716: Add test support requires fork in simple_subprocess (gh-103717)

add requires fork as test case depends on this

Author: itsankitkp

Lib/test/test_socketserver.py

@@ -56,7 +56,7 @@ class ForkingUnixDatagramServer(socketserver.ForkingMixIn,
                                     socketserver.UnixDatagramServer):
         pass
 
-
+@test.support.requires_fork()
 @contextlib.contextmanager
 def simple_subprocess(testcase):
     """Tests that a custom child process is not waited on (Issue 1540386)"""