pythongh-84559: skip the test when no multiprocessing (wasm, etc) (python#101530)

skip test when no _multiprocessing (wasm, etc)

Author: gpshead

Lib/test/test_multiprocessing_defaults.py

@@ -4,10 +4,13 @@
 import multiprocessing
 from multiprocessing.context import DefaultForkDeprecationWarning
 import sys
-from test.support import threading_helper
+from test.support import import_helper, threading_helper
 import unittest
 import warnings
 
+# Skip tests if _multiprocessing wasn't built.
+import_helper.import_module('_multiprocessing')
+
 
 def do_nothing():
     pass