Updated GroupFilterGambit to prevent hidden groups being visible wher…

[ajaypayne]

…e they shouldn't be and to ensure that only the selected groups are returned on a search. #2559

**Fixes #2559 **

Changes proposed in this pull request:
modify db query to use and instead of or

Screenshot
n/a

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[askvortsov1]

Thank you very much for this! The code looks good, I just have a few comments on test cases. It might also be good to test that multiple comma-separated groups can be included in the search.

[askvortsov1]

Any chance we could include a test case for multiple groups in the same query? IE:

'q' => 'group:admin,mod'

or even
'q' => 'group:admin,4'

[ajaypayne]

Sorry, I missed that comment the first time around. Adding now.

[askvortsov1]

Excellent, thank you so much!