[dsymutil] Require darwin in fat binary test

dsymutil uses lipo(1) to build the fat binary, which it invokes as a
process. For that, we need to only run this test on darwin systems.

Should fix: http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-ubuntu/builds/181/steps/test-check-all/logs/stdio