Fix some platform specific paths in TestSettings.py.

We were referring to hardcoded paths /bin/ls and /bin/cat.  For
the purposes of this test, the actual value it's set to doesn't
matter, and it might as well be a non-existent path.  All that
matters is that the before and after values have to match, and
that trailing whitespace is stripped.  On Windows FileSpec
(correctly) converts /bin/ls to D:\bin\ls though, so the before
and after values won't match.  So this patch just correctly builds
up a valid path in a platform-agnostic manner, and verifies that
it matches before and after the set.

llvm-svn: 226625