Clean up and restructure specs of secrets push check

What does this MR do and why?

Cleans up some specs as outlined in this issue: Follow-up from "Secrets push check uses project... (#460011 - closed).

Also address this feedback:

question: would it be more explicit to expect that we never call ::Gitlab::SecretDetection::Scan#secrets_scan? It looks like there are other ways for the method to return nil and we're explicitly hoping it doesn't run the scan.

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