Remove direct setup of `shared_runners_seconds` in specs

What does this MR do and why?

Related to #277448 (closed)

This MR builds on top of !71403 (merged)

In this MR we introduce a method Namespace#unsafe_set_ci_minutes_used! which allows us to override the CI minutes used in specs. This method is will only be used temporarily while we transition from the legacy CI minutes tracking using shared_runners_seconds to the new monthly CI minutes tracking that uses Ci::Minutes::NamespaceMonthlyUsage model.

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