Simplify testing array contents

We were testing for the size and the content of the elements when we
could test for the array itself.

spec/models/community_spec.rb

@@ -21,9 +21,7 @@ RSpec.describe Community, type: :model do
       create(:comment, commentable: topic1, author: user2)
       topic2 = create(:topic, community: community, author: user2)
 
-      expect(community.participants).to include(user1)
-      expect(community.participants).to include(user2)
-      expect(community.participants).to include(proposal.author)
+      expect(community.participants).to match_array [user1, user2, proposal.author]
     end
   end
 end