[llvm-objdump][test] Fix --prefix tests for system-windows

Merging directories and files may produce different results on different
platforms.

Merging "./Inputs" and "source-interleave-x86_64.c" will use different
separators in POSIX and Windows.

Dedicated tests are needed for dealing with removing trailing separators
for POSIX (consider only '/') and Windows (consider '/' and '\').

Fixes D85024.
Fixes PR46368.

Reviewed By: jhenderson, MaskRay

Differential revision: https://reviews.llvm.org/D95513