test, trezor: Check for warnings rather than needs_passphrase_sent

Since bitcoin-core#644, not providing a passphrase to a trezor that has passphrases
enabled will only result in a warning rather than setting
`needs_passphrase_set` to false.

test/test_trezor.py

@@ -340,7 +340,7 @@ def test_passphrase(self):
         result = self.do_command(self.dev_args + ['enumerate'])
         for dev in result:
             if dev['type'] == 'trezor' and dev['path'] == 'udp:127.0.0.1:21324':
-                self.assertTrue(dev['needs_passphrase_sent'])
+                self.assertIn("warnings", dev)
                 break
         else:
             self.fail("Did not enumerate device")