[libc] Don't configure test and fuzzer when -DLLVM_INCLUDE_TESTS=OFF

Summary:  Fixes problem reported in D74397#1935863

Reviewers: nathanchance, gchatelet, sivachandra, MaskRay

Reviewed By: sivachandra, MaskRay

Subscribers: mgorny, MaskRay, tschuett

Differential Revision: https://reviews.llvm.org/D76577