Problem:
The :intro fails to display in ui2 because wait_return(true) triggers a full
redraw. Furthermore statuslines are visible when using :intro
Solution:
Replace wait_return() with plain_vgetc(). This works with old and ui2 while also removing the unnecessary statusline and "press ENTER" prompt.
Updates tests to work with new behaviour.
Signed-off-by: ashab-k <ashabkhan2000@gmail.com>