[tests] Fix p2p_sendheaders race

p2p_sendheaders has a race in part 1.3.

part 1.2 sends a block to the node over the 'test_node' connection, but
doesn't wait for an inv to be received on the 'inv_node' connection. If
we get to part 1.3 before that inv has been received, then the
subsequent call to check_last_inv_announcement could fail.

Author: jnewbery

test/functional/p2p_sendheaders.py

@@ -306,6 +306,7 @@ def test_nonnull_locators(self, test_node, inv_node):
                 test_node.clear_block_announcements()  # since we requested headers...
             elif i == 2:
                 # this time announce own block via headers
+                inv_node.clear_block_announcements()
                 height = self.nodes[0].getblockcount()
                 last_time = self.nodes[0].getblock(self.nodes[0].getbestblockhash())['time']
                 block_time = last_time + 1
@@ -315,6 +316,7 @@ def test_nonnull_locators(self, test_node, inv_node):
                 test_node.wait_for_getdata([new_block.sha256])
                 test_node.send_message(msg_block(new_block))
                 test_node.sync_with_ping()  # make sure this block is processed
+                wait_until(lambda: inv_node.block_announced, timeout=60, lock=mininode_lock)
                 inv_node.clear_block_announcements()
                 test_node.clear_block_announcements()