Add specs for job token scope blocking artifacts download

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/336105

This MR only adds some specs that ensures that the job token scope blocks unauthorized artifacts downloads. It was originally believed that it wasn't working correctly during the initial implementation and tests.

The specs added are around the expected behaviour:

Given that project B is not in the project A's job token scope, the artifact download should not be allowed. It should be allowed only if project B is added to A's job token scope.

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