[llvm-objcopy] Add additional testing for various cases

This patch adds a number of tests to test various cases not covered by
existing tests. All of them work correctly, with no need to change
llvm-objcopy itself, although some do indicate possible areas for
improvement.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D61727

llvm-svn: 360422