test: Extends wait_for_getheaders so a specific block hash can be checked

Previously, `wait_for_getheaders` would check whether a node had received **any**
getheaders message. This implied that, if a test needed to check for a specific block
hash within a headers message, it had to make sure that it was checking the desired message.
This normally involved having to manually clear `last_message`. This method, apart from being
too verbose, was error prone, given an undesired `getheaders` would make tests pass.

This adds the ability to check for a specific block_hash within the last `getheaders` message.

test/functional/mining_basic.py

@@ -308,7 +308,7 @@ class MiningTest(BitcoinTestFramework):
 
         # Should ask for the block from a p2p node, if they announce the header as well:
         peer = node.add_p2p_connection(P2PDataStore())
-        peer.wait_for_getheaders(timeout=5)  # Drop the first getheaders
+        peer.wait_for_getheaders(timeout=5, block_hash=block.hashPrevBlock)
         peer.send_blocks_and_test(blocks=[block], node=node)
         # Must be active now:
         assert chain_tip(block.hash, status='active', branchlen=0) in node.getchaintips()