Take advantage of musl's testsuite

Musl's libc-test should be helpful for testing the library, once it is more complete.

I've previously ported libc-test to partially run with Redox's newlib standard library.

@dlrobertson Noted in the chat that one of the bigger issues to make this work well is #44. I'm planning to put a little effort into that in the next day or so, but I don't know how much time I'll have.