[llvm-objdump] enhance test case for mach-o -private-headers

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220120 91177308-0d34-0410-b5e6-96231b3b80d8
[llvm-objdump] don't test timestamp dump as that is time zone dependent

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220123 91177308-0d34-0410-b5e6-96231b3b80d8