pytest: Chaning startup sentinel for bitcoind.

By looking for 'Done loading' in the log output we should actually be
called after `SetRPCWarmupFinished` in bitcoind. Only then is it safe
to make RPC calls. This resulted in the test suite being a bit flaky.

tests/utils.py

@@ -139,7 +139,8 @@ class BitcoinD(TailableProc):
 
     def start(self):
         TailableProc.start(self)
-        self.wait_for_log("dnsseed thread exit", timeout=10)
+        self.wait_for_log("Done loading", timeout=10)
+
         logging.info("BitcoinD started")