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/p2p_segwit.py

@@ -191,14 +191,13 @@ class TestP2PConn(P2PInterface):
     def announce_block_and_wait_for_getdata(self, block, use_header, timeout=60):
         with p2p_lock:
             self.last_message.pop("getdata", None)
-            self.last_message.pop("getheaders", None)
         msg = msg_headers()
         msg.headers = [CBlockHeader(block)]
         if use_header:
             self.send_message(msg)
         else:
             self.send_message(msg_inv(inv=[CInv(MSG_BLOCK, block.sha256)]))
-            self.wait_for_getheaders(timeout=timeout)
+            self.wait_for_getheaders(block_hash=block.hashPrevBlock, timeout=timeout)
             self.send_message(msg)
         self.wait_for_getdata([block.sha256], timeout=timeout)