[llvm-objdump][test] Improve testing of some switches #2

This patch focuses on adding additional testing for the --source switch.
For reference, the source-interleave-x86_64.ll test file has been split
into two parts - the input (shared with the other tests) and the test
itself.

Reviewed by: MaskRay, rupprecht, grimar

Differential Revision: https://reviews.llvm.org/D61996

llvm-svn: 361479