Add support for running automated tests with llvm.

Update nightly to support new `-llvm` flag, which adds `--llvm` to `start_test` compopts. Also add new
testing script that runs the hellos and extern/ferguson tests (these were requested by @mppf).
Yeah, I agree. I considered adding a new env var that nightly would use to determine which tests to run, but decided that was about the same as doing this.