Automate MDLQA-145 - Groups may be created automatically by a teacher

As described in MDLQA-145, groups may be created automatically by a teacher.

There is already a group/tests/behat/auto_creation.feature, a new scenario should be added to cover MDLQA-145. Probably the best solution is to add a Background section as most of the initial context setup will be the same.

The test should be backported to 2.5
Copying steps from linked QA here:-

This test requires a course in which at least 7 students are enrolled.

1. Login as a teacher, access a course and follow the groups link in the course administration settings.
2. Click the auto-create groups button.
3. Set the number of groups to 3 then click the preview button.
4. Check that users are organised into groups correctly.
5. Set the members per group to 3 then click the preview button.
6. Check again that users are organised into groups correctly.
7. Tick the 'Prevent last small group' checkbox then click the preview button.
8. Check that the additional student has been allocated into another group so that all groups have at least 3 students.