[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.
Do you mean, additional to `-i viminfo` or instead of it? If instead: sure, that should also work. I came up with the `--cmd` solution for the functional tests after I fixed this, so that's why it's different here. Let me just retest it, to be 100% sure that it also fixes the bug. Also: should I also include https://github.com/three-comrades/neovim/commit/3b91e891bdc326c1fb9a18ff1461f201fec2fcdb here?