Fix spec that was dependent on GitLab.com state

What does this MR do and why?

The submodule test in spec/features/projects_spec.rb clicked on the link to the gitlab-grack submodule, which directed the browser to a repository on GitLab.com. The test then proceeded to check the reference selector of GitLab.com, but this can change depending on the version deployed.

To avoid testing against GitLab.com, adjust the test to ensure that a link is available pointing to the right location.

Relates to gitlab-org/quality/engineering-productivity/master-broken-incidents#412 (closed)

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