[MM-56031] Add DeleteSidebarCategory method and channel category API …

…tests (mattermost#29161)

* 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 <[email protected]>
Co-authored-by: Ben Schumacher <[email protected]>
Co-authored-by: Mattermost Build <[email protected]>