pytest: fix flaky test in test_htlc_sig_persistence

We can shutdown before the tx hits bitcoind.

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

tests/test_lightningd.py

@@ -3282,6 +3282,8 @@ class LightningDTests(BaseLightningDTests):
 
         # This should reload the htlc_sig
         l2.rpc.dev_fail(l1.info['id'])
+        # Make sure it broadcasts to chain.
+        l2.daemon.wait_for_log('sendrawtx exit 0')
         l2.stop()
         l1.bitcoin.rpc.generate(1)
         l1.start()