Makefile: add env var to enable args to test cmd

[meowsbits]

This allows make command to handle passing -coverage and other arbitrary
flags to the go command.

Usage:

> make testargs=foo test

eg.

> make testargs=./core test # Test only ./core package
> make testargs='-coverage' test # Include coverage info

[karalabe]

I'm not really sure we want to make the Makefile any smarter. Originally all our builds were script driven and it became unusable, so we've rewritten the whole thing in Go, and made CI, builders, testers just call a single Go command.

I know that this step just makes calling that Go command a bit easier, but wouldn't it be simpler for yourself to just call go run build/ci.go <args> yourself instead of extending make?

[karalabe]

Ping @fjl if you feel one way or another.

[meowsbits]

Yea, I hear on the go vs make command. This change is related to #20298, and I just found it useful to be able to 'live in' the make command during development instead of switching back and forth, or writing custom scripts that duplicate Makefile functionality.

[meowsbits]

But my heart's not set on it either way. It just seemed like a very tiny low-maintenance self-documenting addition that opens a lot of doors for the command.

[karalabe]

opens a lot of doors for the command

That's exactly what we'd like to avoid :)

Make is mostly a crutch to help newcomers, but it's not the recommended way to work with the code base. It was a lot more important way back when our builders were based on scripts (and we had C dependencies). Now almost everything is Go and with Go modules coming, even more can be deleted. I think the general direction is getting rid of all non-Go workflows, so I'd rather not make make more useful :P