If Nvim is in a "Press ENTER..." screen before the `get_vvar()` call, the test will hang.
Fixes #3676. Reviewed-by: Marco Hinz <mh.codebro@gmail.com>
- change approach for test 1: screen:expect() instead of assert() - use execute() instead of command() - 2 new tests that check none and wrong input for :oldfiles! Helped-by: @fwalch Helped-by: @tarruda Helper-by: @justinmk