[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

Author: abrachet

libc/CMakeLists.txt

@@ -32,5 +32,7 @@ add_subdirectory(utils)
 # and libraries potentially draw from the components present in all
 # of the other directories.
 add_subdirectory(lib)
-add_subdirectory(test)
-add_subdirectory(fuzzing)
+if(LLVM_INCLUDE_TESTS)
+  add_subdirectory(test)
+  add_subdirectory(fuzzing)
+endif()