Add some guides of writing go test

[FlyingOnion]

I still have some confusions about writing units. I hope the guide could contains some suggestions of writing tests. Maybe you could consider:

1. how to define cases?
2. should we define both "pass" and "not pass" cases? or just "pass" cases?
3. how to test errors (or fatals/panics, if needed)?
4. (maybe difficult) some guides about mock or noop servers/clients.

Thank you.