[tests] sync_with_ping should assert that ping hasn't timed out

sync_with_ping currently returns false if the timeout expires, and it is
the caller's responsibility to fail the test. However, none of the tests
currently assert on sync_with_ping()'s return code. This commit adds an
assert to sync_with_ping so the test will fail if the timeout expires.

This commit also removes all the duplicate implementations of
sync_with_ping() from the individual tests.

test/functional/p2p-versionbits-warning.py

@@ -46,21 +46,6 @@ class TestNode(NodeConnCB):
     def on_pong(self, conn, message):
         self.last_pong = message
 
-    # Sync up with the node after delivery of a block
-    def sync_with_ping(self, timeout=30):
-        self.connection.send_message(msg_ping(nonce=self.ping_counter))
-        received_pong = False
-        sleep_time = 0.05
-        while not received_pong and timeout > 0:
-            time.sleep(sleep_time)
-            timeout -= sleep_time
-            with mininode_lock:
-                if self.last_pong.nonce == self.ping_counter:
-                    received_pong = True
-        self.ping_counter += 1
-        return received_pong
-
-
 class VersionBitsWarningTest(BitcoinTestFramework):
     def __init__(self):
         super().__init__()