Attempt to fix MacOS test failures

...or at least produce more useful output if they fail

Author: duaneg

Lib/test/_test_multiprocessing.py

@@ -6818,9 +6818,12 @@ def test_std_streams_flushed_after_preload(self):
                     print('stderr', file=sys.stderr)''')
 
         name = os.path.join(os.path.dirname(__file__), 'mp_preload_flush.py')
-        env = {'PYTHONPATH': ":".join(sys.path)}
+        env = {'PYTHONPATH': self._temp_dir}
         rc, out, err = test.support.script_helper.assert_python_ok(name, **env)
         self.assertEqual(rc, 0)
+
+        # We want to see all the output if it isn't as expected
+        self.maxDiff = None
         self.assertEqual(out.decode().rstrip(), 'stdout')
         self.assertEqual(err.decode().rstrip(), 'stderr')
 

Lib/test/mp_preload_flush.py

@@ -1,7 +1,12 @@
 import multiprocessing
+import sys
+
 if __name__ == '__main__':
+    assert 'a' not in sys.modules
     multiprocessing.set_forkserver_preload(['a'])
     for _ in range(2):
         p = multiprocessing.Process()
         p.start()
         p.join()
+else:
+    assert 'a' in sys.modules