Community tests -> Optimisation: get rid of unnecessary app launches when possible

[anastasiyaig]

Review community related tests and get rid of unnecessary app launches when possible. Other words, if we can do tests in the same community - let's do that.

Be careful so tests wont become self-dependant

[Valentina1133]

I will work on it in several PRs, will be fixed file by file to avoid too many changes in one PR and for more convenient review.

test_communities.py - #526 - merged
test_communities_categories.py - #529 - merged
test_communities_channels.py - #533 - merged
test_communities_permissions.py - already done, separate ticket for stabilisation
test_communities_screens_overview.py - no changes needed
test_communities_pin_and_unpin_messages.py and test_join_community.py - PR #556

[Valentina1133]

All done