invalidtag exception for gcm

This exception is probably not safe. It depends on the assumption that
if ERR_get_error returns a 0 then it is an AEAD tag error.
diff --git a/tests/hazmat/primitives/utils.py b/tests/hazmat/primitives/utils.py
index 8df02e7..39f5ae8 100644
--- a/tests/hazmat/primitives/utils.py
+++ b/tests/hazmat/primitives/utils.py
@@ -7,7 +7,7 @@
 from cryptography.hazmat.primitives import hashes, hmac
 from cryptography.hazmat.primitives.ciphers import Cipher
 from cryptography.exceptions import (
-    AlreadyFinalized, NotYetFinalized, AlreadyUpdated,
+    AlreadyFinalized, NotYetFinalized, AlreadyUpdated, InvalidTag,
 )
 
 from ...utils import load_vectors_from_file
@@ -95,7 +95,7 @@
         decryptor = cipher.decryptor()
         decryptor.add_data(binascii.unhexlify(aad))
         actual_plaintext = decryptor.update(binascii.unhexlify(ciphertext))
-        with pytest.raises(AssertionError):
+        with pytest.raises(InvalidTag):
             decryptor.finalize()
     else:
         cipher = Cipher(