Change binary path in libc test cmake file from relative to absolute (#148315)

For some reason cmake usually turns this path into an absolute path, but
sometimes
it doesn't do it and stays as a relative path, which means the command
will fail.
Specify it as an absolute path always so this doesn't happen any more.

Author: amykhuang

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -327,7 +327,7 @@ function(create_libc_unittest fq_target_name)
   if(NOT LIBC_UNITTEST_NO_RUN_POSTBUILD)
     add_custom_target(
       ${fq_target_name}
-      COMMAND ${LIBC_UNITTEST_ENV} ${CMAKE_CROSSCOMPILING_EMULATOR} ${fq_build_target_name}
+      COMMAND ${LIBC_UNITTEST_ENV} ${CMAKE_CROSSCOMPILING_EMULATOR} ${CMAKE_CURRENT_BINARY_DIR}/${fq_build_target_name}
       COMMENT "Running unit test ${fq_target_name}"
       DEPENDS ${fq_build_target_name}
     )