Hide share link menu before copying the share link in acceptance tests

The (old) Firefox version used in the acceptance tests does not properly
render the share link menu. As the menu is taller than it should
sometimes it covers the copy link button, so it is not possible to click
it without hiding the share link menu. Moreover, in those cases the
share menu button is also covered by the share menu, so the menu needs
to be closed by pressing the "Esc" key.

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