Add failing test for `gf`

Ref: https://github.com/junegunn/vader.vim/issues/30

    % vim -Nu vimrc -c 'Vader! regression/gf.vader'
    …
    Starting Vader: 1 suite(s), 2 case(s)
      Starting Vader: …/vim/neobundles/vader/test/regression/gf.vader
        (1/2) [  GIVEN]
        (1/2) [     DO] gf
        (2/2) [  GIVEN]
        (2/2) [EXECUTE] check
        (2/2) [EXECUTE] (X) Content has not been changed to Given.
      Success/Total: 1/2
    Success/Total: 1/2 (assertions: 1/2)