[lit] Make it possible for the lit test suite to pass with
`FILECHECK_OPTS=-v` set in the environment.

Follow up to r350850 as requested by Joel E. Denny in
https://reviews.llvm.org/D56541 .

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350854 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed