PROTON-2115 fix typo in tests/py/test_unittest.py breaking fd-limit test (#220)

diff --git a/tests/py/test_unittest.py b/tests/py/test_unittest.py
index bc372f6..8073ce2 100644
--- a/tests/py/test_unittest.py
+++ b/tests/py/test_unittest.py
@@ -50,7 +50,7 @@
         if condition:
             return skip(reason)
         return lambda f: f
-    unittest.skipUnless = skipIf
+    unittest.skipIf = skipIf
 
 if not hasattr(unittest, "skipUnless"):
     def skipUnless(condition, reason):