Fix bad inheritance in test_subprocess that led to a number of tests being executed twice.

Lib/test/test_subprocess.py

@@ -1702,7 +1702,7 @@ class CommandsWithSpaces (BaseTestCase):
         self.with_spaces([sys.executable, self.fname, "ab cd"])
 
 
-class ContextManagerTests(ProcessTestCase):
+class ContextManagerTests(BaseTestCase):
 
     def test_pipe(self):
         with subprocess.Popen([sys.executable, "-c",