Revert "llvm-cov: Remove an overly system specific test"

This fixes a call to sys::fs::equivalent that should've been to
CodeCoverageTool::equivalentFiles, which lets us restore the test of
r217476 that was removed in r217478.

This reverts r217478, but the test works this time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217646 91177308-0d34-0410-b5e6-96231b3b80d8