test: rename CBlockHeader .hash -> .hash_hex for consistency

Note that we unfortunately can't use a scripted diff here, as the
`.hash` symbol is also used for other instances (e.g. CInv).

test/functional/test_framework/messages.py

@@ -748,7 +748,7 @@ class CBlockHeader:
         return r
 
     @property
-    def hash(self):
+    def hash_hex(self):
         """Return block header hash as hex string."""
         return hash256(self._serialize_header())[::-1].hex()