Wait for the shared link to be set in the acceptance tests

When clicking on "Share link" in the "Sharing" tab of the Files app an
input field with the link appears. That input field already exists in
the DOM, although empty, before clicking on "Share link", and when that
is done the proper value is set and then the input field is shown.

In the acceptance tests "getValue()" can return the value of hidden
elements too, so as long as an element exists its value is returned
without waiting for the field to be visible. Due to this if the test
code runs too fast the "I write down the shared link" step could be
executed before the proper value was set, so the shared link got in that
case would be an empty value, and this would lead to failures when the
following steps were executed.

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