Fix a buildbot test failure caused by paths longer than the expected 200 characters.  Replace path buffer size with the standard PATH_MAX from limits.h. I think this is a very safe change, but please revert if it causes problems.

git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@190918 91177308-0d34-0410-b5e6-96231b3b80d8