Create behat tests for groups filter

The forum summary report groups filter requires behat tests.

Some cases that need to be handled include:

1. Forums in courses containing no groups.
2. All groups selected by default.
3. All groups can be selected/cleared together, and selecting all will set "Groups (all)" on the filter button.
4. Selecting some groups only.
5. Selecting a mixture of some groups and "No groups" (users not in any groups).
6. Selecting "No groups only".
7. Submitting with no groups checkboxes ticked.
8. Selecting a group which contains no users.

Where relevant, tests need to have some intersecting users among groups (eg selecting Group A and Group B, where Teacher 1 is in both groups), to ensure that belonging to 2 selected groups does not cause the data to be duplicated (ie show 4 replies when there were only 2). This will be important when additional changes are made, to ensure no regressions affect the report's data integrity.