Integrate Go cases into build scripts

[tisonkun]

I think basically we can treat Go cases as a standalone module and write a Makefile to it for running linters and tests.

Then we wire it into the x.py script.

[PragmaTwice]

I am thinking whether the makefile is necessary, since it is just a test module and we already have x.py which is a general script for developers.

If there are two scripts that need to be maintained, I think it might cause some confusion. The functionality of the makefile is not orthogonal to our current x.py, they have similiar capabilities.

[tisonkun]

@PragmaTwice you're right. Previously I'm thinking of adding linters for Go code and then build/test can depend on the lint task. Makefile does a good job for task dependencies while implementing it inside a script is awkward. However, now I believe we can run lint in another x.py command and there's no dependency between tasks to handle. So your patch is great.

[tisonkun]

Closed by #832.