Bug 29485: (follow-up) Trying to prevent administration_tasks.t to fail randomly

Bug 29485: (follow-up) Trying to prevent administration_tasks.t to fail randomly

Recreated the problem after run #47
Error while executing command: no such element: Unable to locate element: //*[@id="userid"] at /usr/local/share/perl/5.28.1/Selenium/Remote/Driver.pm line 411.

With this patch I do not longer recreate the failure. It's ugly but,
well, I don't have any other solutions. It seems that the accept_alert
is taking too long and is async (??)

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>