Fixes test failure on darwin introduced by r188274

Reworked how the tests dealt with paths. Instead of removing the full path name,
put a marker in the expected file and replace the marker with the full path
before running diff.

Differential Revision: http://llvm-reviews.chandlerc.com/D1381


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188299 91177308-0d34-0410-b5e6-96231b3b80d8