[Polly] [Tests] Update `lit.cfg` uses of `lit.util.capture` to `subprocess.check_output`

- `lit.util.capture` was removed in `r306625`.
- Replace `lit.util.capture` to `subprocess.check_output` as LLVM did.
- LLVM revision of this change: `https://reviews.llvm.org/D35088`.

Differential Revision: https://reviews.llvm.org/D35255

llvm-svn: 307765