[RFC] Use a local viminfo file when running the old tests.

Because @elmart also encountered the test failure when running the old tests (see #1694), I think it is a good ideat to merge this fix as soon as possible. Thus, I cherry-picked the fix from #1678 which will probably take some more time, until it is ready.
Why not `--cmd "set viminfo+=nnviminfo"`? Just in case...