Issue Description:

Currently run-test requires you to pass the build-directory for the compiler/stdlib to use for the tests, but this is tedious and requires the user to actually know what the name of the target they're building was.

A more ergonomic workflow would be to allow the same flags that build-script accepts to be passed to run-test, and have it derive the compiler from that. I'm not sure what the complete set of flags relevant flags is, but -d, -r, and -R are probably the most important. More complex scenarios can always fall-back to the current workflow.