make-check-very-fast

[catamorphism]

It would be nice to have a make target that just builds a few tests that are known to not require any of the standard libraries. That way, for preliminary testing, you could get test results quickly (without having to run them one by one) without having to wait for all the libraries to build.

[brson]

The build prerequisites to start running compiler tests are numerous at the moment, and I accidentally made it worse recently by making librustdoc, etc. prerequisites. I'll fix that shortly.

I think we can easily make it so the target libs only need to build through std before testing. That should be better, but you'll still have to build all the way through stage0. It would be nice if make check-stage0-rpass worked and only required core and std to be built.

[brson]

The prereqs for compile tests are just core and std now.

[catamorphism]

This feels less important to me now that we have bors and you don't
have to run all tests locally before submitting. Closing.