[libc] Remove use of 'assert' from the string tests

Currently we use `assert` as a stand-in for the typical test assertions.
This is because these functions exist outside of the base test class so
we can't use the typical assertion methods. The presence of these
asserts makes it difficult to compile these tests in a standalone
format. This patch removes all occurrences.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149656