tests don't call relibc implementation of string functions.

Created by: jrraymond

I was implementing strpbrk, which calls strcspn. When called from inside my implementation of strpbrk, strcspn was crashing at src/string/src/lib.rs:140: attempt to shift left with overflow on the inputs hello, worlde, aeiouy. However, when I called strcspn from the tests with the same inputs it worked.

So I commented out almost the entire implementaiton of strcspn so it would always return 0. But when I run the tests strcspn still produces the correct results.

I also commented out most of the implementation of strspn and strchr, and they still produce the correct results. So I added back the uniplemented! macro and the tests still work.

I wonder if this is just some quirk of my system?