Fix test_kv_service (#2944)
Fix test_kv_service (#2944)

When we write to an unopened engine, the first chunk will fail with an
error response, and the stream will be closed. Then if we send another
chunk with that stream, we will get an error and the test will
fail. So we fix it by sending only the write head when we just want to
check the error response.
pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv,pingcap/tikv