MDL-43941 Course: Allow AJAX show/hide of activities in hidden and orphaned sections
MDL-43941 Behat: Section visibility tests should not check if activities are visible

It should be possible for activities to be visible even when the section
they are in is hidden.

However, when a section is hidden, all activities in that section are also
then initially hidden. Therefore, this commit splits out the hidden test to
a new test so that the initial state of activities after hiding can still
be tested.