[RFC] Don't read the viminfo file before running the old tests.

With `nocp` set as the default, nvim reads the viminfo file before running the old tests. This can lead to unexpected test failures, like in #1673 .

This PR sets `viminfo=` in the test vimrc file and changes it back to the default value in `dotest.in`. 

Edit: I also thought about `-i NONE`, but test74 explicitly tests some viminfo behavior. 
In the end it doesn't make a big difference. IMO, the busted output for pending tests is even harder to read than the output of `grep -Rn -- --pending test` (at least if `alias grep='grep --color=auto` is used). So we'd just have to be consistent with `--pending` in all tests.