make this test assert the right thing. (#3133)

right now it just always skips
diff --git a/tests/hazmat/primitives/test_x963_vectors.py b/tests/hazmat/primitives/test_x963_vectors.py
index b09d165..4a945d4 100644
--- a/tests/hazmat/primitives/test_x963_vectors.py
+++ b/tests/hazmat/primitives/test_x963_vectors.py
@@ -62,4 +62,5 @@
         xkdf.verify(key, key_data)
 
     def test_unsupported_hash(self, backend):
-        _skip_hashfn_unsupported(backend, DummyHashAlgorithm())
+        with pytest.raises(pytest.skip.Exception):
+            _skip_hashfn_unsupported(backend, DummyHashAlgorithm())