bpo-31904: Port test_cmd_line to VxWorks (#12648)

subprocess.Popen doesn't support preexec on VxWorks.

Lib/test/test_cmd_line.py

@@ -369,6 +369,8 @@ def test_closed_stdout(self):
     # Issue #7111: Python should work without standard streams
 
     @unittest.skipIf(os.name != 'posix', "test needs POSIX semantics")
+    @unittest.skipIf(sys.platform == "vxworks",
+                         "test needs preexec support in subprocess.Popen")
     def _test_no_stdio(self, streams):
         code = """if 1:
             import os, sys