* Add DeleteSidebarCategory method and channel category API tests * Add test to verify channels move to default categories when custom category is deleted * rename channel category tests and remove duplicate context * TestNotifications fix --------- Co-authored-by: Fume <contact@fumedev.com> Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com> Co-authored-by: Mattermost Build <build@mattermost.com>