Add spec_helper where required
What does this MR do and why?
There were several instances where specs were failing as fast_spec_helper
wasn't accessing the necessary classes.
This change addresses those instances, though there are still a couple of Gitlab::Metrics::System
module specs that have additional, unrelated failures.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #390552 (closed)