[llvm-objcopy] Add tests for help messages

This diff slightly reorganizes the tests  and improves 
the test coverage of help messages / error reports.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D46589



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331993 91177308-0d34-0410-b5e6-96231b3b80d8