Add test case for the tag index page

What does this MR do and why?

This is corrective action for the recent incident on tag index page.

corrective action Add a test case that passes a slashed tag name (e.g. v0.1/test) as a route parameter. spec/controllers/projects/tags_controller_spec.rb would be a good place for this.

This prevents the same incident from happening.

This also adds a basic coverage to the feature specs. It seems we don't have any coverage at the moment.

Related https://gitlab.com/gitlab-com/gl-infra/production/-/issues/7069

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