test_lightningd.py: repair flaky test_bitcoin_failure

If bitcoind is still fetching blocks, we might accidentally inject
the failure between getblockhash and getblock.  That's OK, but
it's not the failure we test for.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_lightningd.py

@@ -1118,6 +1118,9 @@ class LightningDTests(BaseLightningDTests):
     def test_bitcoin_failure(self):
         l1 = self.node_factory.get_node(fake_bitcoin_cli=True)
 
+        # Make sure we're not failing it between getblockhash and getblock.
+        sync_blockheight([l1])
+
         self.fake_bitcoind_fail(l1, 1)
 
         # This should cause both estimatefee and getblockhash fail