[qa] Fix bug in compactblocks v2 merge

Bug caused the wait_for_block_announcement to be called on the wrong node,
leading to nondeterminism and occasional test failures. Bug was introduced in
merge commit:

d075479 Merge bitcoin#8882: [qa] Fix race conditions in p2p-compactblocks.py and sendheaders.py

Underlying commits which conflicted were:

27acfc1 [qa] Update p2p-compactblocks.py for compactblocks v2
6976db2 [qa] Another attempt to fix race condition in p2p-compactblocks.py

The first commit changed the test_compactblock_construction function signature
and second commit added code which wasn't updated during the merge to use the
new arguments.

Suhas Daftuar <sdaftuar@chaincode.com> noticed the bug and suggested the fix.

qa/rpc-tests/p2p-compactblocks.py

@@ -300,8 +300,8 @@ def test_compactblock_construction(self, node, test_node, version, use_witness_a
             assert(segwit_tx_generated) # check that our test is not broken
 
         # Wait until we've seen the block announcement for the resulting tip
-        tip = int(self.nodes[0].getbestblockhash(), 16)
-        assert(self.test_node.wait_for_block_announcement(tip))
+        tip = int(node.getbestblockhash(), 16)
+        assert(test_node.wait_for_block_announcement(tip))
 
         # Now mine a block, and look at the resulting compact block.
         test_node.clear_block_announcement()