bpo-38377: Add support.skip_if_broken_multiprocessing_synchronize() (GH-20944)

On Linux, skip tests using multiprocessing if the current user cannot
create a file in /dev/shm/ directory. Add the
skip_if_broken_multiprocessing_synchronize() function to the
test.support module.
diff --git a/Lib/test/test_venv.py b/Lib/test/test_venv.py
index ef6d7bd..d3191ed 100644
--- a/Lib/test/test_venv.py
+++ b/Lib/test/test_venv.py
@@ -16,7 +16,8 @@
 import tempfile
 from test.support import (captured_stdout, captured_stderr, requires_zlib,
                           can_symlink, EnvironmentVarGuard, rmtree,
-                          import_module)
+                          import_module,
+                          skip_if_broken_multiprocessing_synchronize)
 import unittest
 import venv
 from unittest.mock import patch
@@ -357,10 +358,11 @@
         """
         Test that the multiprocessing is able to spawn.
         """
-        # Issue bpo-36342: Instantiation of a Pool object imports the
+        # bpo-36342: Instantiation of a Pool object imports the
         # multiprocessing.synchronize module. Skip the test if this module
         # cannot be imported.
-        import_module('multiprocessing.synchronize')
+        skip_if_broken_multiprocessing_synchronize()
+
         rmtree(self.env_dir)
         self.run_with_capture(venv.create, self.env_dir)
         envpy = os.path.join(os.path.realpath(self.env_dir),