[llvm-objdump][test] Fix --prefix tests for system-windows
Merging directories and files may produce different results on different
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 '\').
Reviewed By: jhenderson, MaskRay
Differential revision: https://reviews.llvm.org/D95513