Read default flag values from files

What does this MR do and why?

Lets us properly use feature flags when a flag has a default value defined in a feature flag definition file but doesn't have a record in the database yet because the flag hasn't been toggled.

• Reads default flag values from definition files during tests if the API doesn't return a value.
• Adds an e2e sanity test that checks if the definition fails can be read.

A job to run the new sanity test will be added in gitlab-org/quality/pipeline-common!191 (merged)

Closes #362182 (closed)

Tasks

• Dequarantine tests put in quarantine in #362182 (closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.