[lit] Make it possible for the lit test suite to pass with `FILECHECK_OPTS=-v` set in the environment.
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@350854 91177308-0d34-0410-b5e6-96231b3b80d8

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