qa: unflake test_rescan_edge_cases

We weren't waiting for lianad to be back to the same height as bitcoind.
Necessarily it could happen that sorted_coins() would return an empty
list.

Fixes #142 (did 50 successful runs of the test)

tests/test_chain.py

@@ -222,6 +222,7 @@ def test_rescan_edge_cases(lianad, bitcoind):
     lianad.stop()
     lianad.start()
     wait_for(lambda: lianad.rpc.getinfo()["rescan_progress"] is None)
+    wait_synced()
     assert coins_before == sorted_coins()
 
     # Lose our state again