qa: Avoid start/stop of the network thread mid-test

test/functional/p2p_compactblocks.py

@@ -788,13 +788,11 @@ class CompactBlocksTest(BitcoinTestFramework):
         assert_equal(int(node.getbestblockhash(), 16), block.sha256)
 
     def run_test(self):
-        # Setup the p2p connections and start up the network thread.
+        # Setup the p2p connections
         self.test_node = self.nodes[0].add_p2p_connection(TestP2PConn())
         self.segwit_node = self.nodes[1].add_p2p_connection(TestP2PConn(), services=NODE_NETWORK|NODE_WITNESS)
         self.old_node = self.nodes[1].add_p2p_connection(TestP2PConn(), services=NODE_NETWORK)
 
-        network_thread_start()
-
         self.test_node.wait_for_verack()
 
         # We will need UTXOs to construct transactions in later tests.