[libc] add integration tests for scudo in libc

This change adds tests to make sure that SCUDO is being properly
included with llvm libc. This change also adds the toggles to properly
use SCUDO, as GWP-ASan is enabled by default and must be included for
SCUDO to function.

Reviewed By: sivachandra, hctim

Differential Revision: https://reviews.llvm.org/D106919