Behat: Improve background set-up for groups

[Rebecca Blundell]

Certain options such as
- Group category
- Group invitation (group_member_invite)
are not accessible as background steps.

To improve:
1. Check which other functionality could be added to background steps for groups to make the tests easier to manage.
2. Add functionality and refactor tests to use it.

Note that this is a wishlist item reported as a reminder that improvements can be made. It will be edited if other options are found.