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).
Seems like a hacky way to get the tests you want. Although I can't think of a better way without adding more cruft like we do for mulitlocale