unittest: Add dummy TestCase.subTest() context manager.

Just runs "subtests" in the scope of the main TestCase.

Signed-off-by: Paul Sokolovsky <pfalcon@users.sourceforge.net>

python-stdlib/unittest/unittest.py

@@ -29,10 +29,21 @@ class AssertRaisesContext:
         return False
 
 
+class NullContext:
+    def __enter__(self):
+        pass
+
+    def __exit__(self, a, b, c):
+        pass
+
+
 class TestCase:
     def __init__(self):
         pass
 
+    def subTest(self, msg=None, **params):
+        return NullContext()
+
     def skipTest(self, reason):
         raise SkipTest(reason)