trezor: add ping for self-test

sshagent/tests/test_trezor.py

@@ -42,6 +42,10 @@ class ConnectionMock(object):
         result.node.public_key = PUBKEY
         return result
 
+    def ping(self, msg):
+        assert not self.closed
+        return msg
+
 
 class FactoryMock(object):
 

sshagent/trezor/client.py

@@ -25,6 +25,8 @@ class Client(object):
         log.debug('revision : %s', binascii.hexlify(f.revision))
 
     def __enter__(self):
+        msg = 'Hello World!'
+        assert self.client.ping(msg) == msg
         return self
 
     def __exit__(self, *args):