Fix mach-o tests on Windows

The tests assume the path separator is '/', but if you run
them on Windows it is '\'. As a result the tests are failing
on Windows. This should be the minimal change to make these
tests to pass on Windows platform.

Differential Revision: http://reviews.llvm.org/D4710

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@214990 91177308-0d34-0410-b5e6-96231b3b80d8