[tests] Fix feature_block flakiness

feature_block.py occasionally fails on Travis. I believe this is due to
a a race condition when reconnecting to bitcoind after a subtest that
expects disconnection. If the test runs ahead and sends the INV for the
subsequent test before we've received the initial sync getheaders, then
we may end up sending two headers messages - one as a response to the
initial sync getheaders and one in response to the INV getheaders. If
both of those headers fail validation with a DoS score of 50 or higher,
then we'll unexpectedly be disconnected.

There is only one validation failure that has a DoS score bewteen 50 and
100, which is high-hash. That's why the test is failing immediately
after the "Reject a block with invalid work" subtest.

Fix is to wait for the initial getheaders from the peer before we
start populating our blockstore. That way we won't have any invalid
headers to respond to it with.

test/functional/feature_block.py

@@ -1306,7 +1306,13 @@ class FullBlockTest(BitcoinTestFramework):
         self.nodes[0].disconnect_p2ps()
         self.nodes[0].add_p2p_connection(P2PDataStore())
         network_thread_start()
-        self.nodes[0].p2p.wait_for_verack()
+        # We need to wait for the initial getheaders from the peer before we
+        # start populating our blockstore. If we don't, then we may run ahead
+        # to the next subtest before we receive the getheaders. We'd then send
+        # an INV for the next block and receive two getheaders - one for the
+        # IBD and one for the INV. We'd respond to both and could get
+        # unexpectedly disconnected if the DoS score for that error is 50.
+        self.nodes[0].p2p.wait_for_getheaders(timeout=5)
 
     def sync_blocks(self, blocks, success=True, reject_code=None, reject_reason=None, request_block=True, reconnect=False, timeout=60):
         """Sends blocks to test node. Syncs and verifies that tip has advanced to most recent block.