Skip forkserver preload tests on platforms without fork support

Add skip decorators to exclude test_preload.py on Android, iOS,
WASI, and other platforms that don't support fork, using the
existing has_fork_support check from test.support.

Author: gpshead

Lib/test/test_multiprocessing_forkserver/test_preload.py

@@ -5,8 +5,11 @@
 import sys
 import unittest
 
+from test.support import has_fork_support
+
 
 @unittest.skipIf(sys.platform == "win32", "forkserver not available on Windows")
+@unittest.skipUnless(has_fork_support, "requires working os.fork()")
 class TestForkserverPreload(unittest.TestCase):
     """Tests for forkserver preload functionality."""