[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. 
:+1: