MDL-76733 mod_assign: Tidy up availability behat tests
This feature had lots of small issues and it made sense to fix it whilst
investigating a query:
* most of the steps do not require JavaScript
* it uses the UI to set an admin setting, for every scenario:
** only 3-4 of the scenarios actually test that setting
** it is very slow to do it his way when we have a generator step we can use
* we create two assignments in the Background, but each test only uses one of them
* we create the assignments in the Background with a generator, but
update them to modify various settings in each Scenario using the UI
when we should just create one assignment for each test and set it up
correctly for that Scenario