disable blowfish in commoncrypto backend for key lengths under 64-bit (#3040)

This is due to a bug in CommonCrypto present in 10.11.x. Filed as
radar://26636600
diff --git a/tests/hazmat/primitives/utils.py b/tests/hazmat/primitives/utils.py
index 8705cdc..d0e87a7 100644
--- a/tests/hazmat/primitives/utils.py
+++ b/tests/hazmat/primitives/utils.py
@@ -47,6 +47,11 @@
 
 
 def encrypt_test(backend, cipher_factory, mode_factory, params):
+    if not backend.cipher_supported(
+        cipher_factory(**params), mode_factory(**params)
+    ):
+        pytest.skip("cipher/mode combo is unsupported by this backend")
+
     plaintext = params["plaintext"]
     ciphertext = params["ciphertext"]
     cipher = Cipher(