[libc] Make string tests compatible with the Fuchsia build

Some test code was doing loose conversions caught by compiler
warnings in  the Fuchsia build.  This included duplicated code
in a few tests that was reconsolidated with the existing header
file copy of the same functions.

The MemoryMatcher abstraction presumes gtest-style matcher support,
which is not available in Fuchsia's zxtest library.  It's avoided
in favor of simpler memory-comparing assertions.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D146343