test: Add protocol_err test for malformed unidirectional message
test: Add protocol_err test for malformed unidirectional message

- test_protocol_err: Add unidirectional message with non-MINIRPC_PENDING
status code

- test_protocol_err: Before closing connections, do synchronous pings on
both of them to make sure the event queues are flushed.  Otherwise, some
ioerr events might be squashed during shutdown and the expected_ioerrs check
could fail.

cmusatyalab/minirpc,cmusatyalab/minirpc