Improve Project Super Sidebar specs

With !131915 (merged) we are switching some of the Project Sidebar related specs to run from in Project in a user namespace to run in a Project in a group namespace. This kind of shows a test gap. We probably should run all project sidebar related specs in both contexts.

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The following discussion from !131915 (merged) should be addressed:

• @leipert started a discussion: (+1 comment)

  @mwoolf This actually shows a big gap in our Navigation specs. We probably should move all the project specific specs to be shared rspec examples and execute them once for projects part of a group namespace and once for projects part of a group namespace... But that surely can be done in a follow-up. cc/ @gitlab-org/manage/foundations/engineering