test_onion.py: drop repeated sha calculation

Author: ajtowns

test/test_onion.py

@@ -174,8 +174,8 @@ def get_ecdh_secrets(cls, sec, pkey_x, pkey_y):
 
         enckey = cls.tweak_sha(sec, b'\x00')[:16]
         hmac   = cls.tweak_sha(sec, b'\x01')
-        iv     = cls.tweak_sha(sec, b'\x02')[:16]
-        pad_iv = cls.tweak_sha(sec, b'\x02')[16:]
+        ivs    = cls.tweak_sha(sec, b'\x02')
+        iv, pad_iv = ivs[:16], ivs[16:]
 
         return enckey, hmac, iv, pad_iv