[llvm-readobj]Test basic command-line handling

There was no real testing for llvm-readobj/llvm-readelf's behaviour
under various bad inputs and command-line switches. This patch adds some
testing of this, along with basic testing of --version and --help.

Reviewed by: MaskRay, grimar

Differential Revision: https://reviews.llvm.org/D58455

llvm-svn: 354567