Fix clicks on actions menu of non opaque file rows in acceptance tests

When a row is added to the file list the opacity of the file row is
animated from transparent to fully opaque. As the file actions menu is a
descendant of the row but overflows it when the row is not fully opaque
clicks on the menu entries "fall-through" and are received instead by
the rows behind.

The opacity animation is a CSS animation, and it is not possible to know
if a row is appearing or not except from its opacity (the row will have
the "appear" CSS class even after the animation ended). Therefore it
should be waited until the row of the file is fully opaque before using
the menu.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>