refactor: test: improve wait_for{header,merkleblock} interface

The interfaces for the methods wait_for_header() and wait_for_merkleblock() are
changed to take a hex string instead of an integer, improving type safety and
removing the burden from the caller to always do the transformation via
`int(...)`. As suggested by MarcoFalke in
https://github.com/bitcoin/bitcoin/pull/18593#discussion_r407062253

test/functional/test_framework/mininode.py

@@ -393,7 +393,7 @@ class P2PInterface(P2PConnection):
             last_headers = self.last_message.get('headers')
             if not last_headers:
                 return False
-            return last_headers.headers[0].rehash() == blockhash
+            return last_headers.headers[0].rehash() == int(blockhash, 16)
 
         wait_until(test_function, timeout=timeout, lock=mininode_lock)
 
@@ -403,7 +403,7 @@ class P2PInterface(P2PConnection):
             last_filtered_block = self.last_message.get('merkleblock')
             if not last_filtered_block:
                 return False
-            return last_filtered_block.merkleblock.header.rehash() == blockhash
+            return last_filtered_block.merkleblock.header.rehash() == int(blockhash, 16)
 
         wait_until(test_function, timeout=timeout, lock=mininode_lock)