[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.
Could you expand this comment to explain what the platform-specific behaviour is, please? (I'm guessing it's related to the trailing separator comment below).