[libc] Enable the 'string' unit tests to be hermetic

This changes over the string unit tests to the new `add_libc_test`
framework. The one test that wasn't changes was the memory utils because
it overran the static buffer used for the hermetic test's bump pointer.

Depends on D149656

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149662