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/p2p_invalid_locator.py

@@ -34,7 +34,7 @@ class InvalidLocatorTest(BitcoinTestFramework):
             msg.locator.vHave = [int(node.getblockhash(i - 1), 16) for i in range(block_count, block_count - (MAX_LOCATOR_SZ), -1)]
             node.p2p.send_message(msg)
             if type(msg) == msg_getheaders:
-                node.p2p.wait_for_header(int(node.getbestblockhash(), 16))
+                node.p2p.wait_for_header(node.getbestblockhash())
             else:
                 node.p2p.wait_for_block(int(node.getbestblockhash(), 16))