Resolves#45220 (one of several PRs to achieve removing "testing"
package as dependency in production binary)
## Testing
- [x] QA'd all new/changed functionality manually.
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## Summary by CodeRabbit
* **Tests**
* Switched many tests to use a dedicated MySQL test helper package and
consolidated test-only utilities for datastore setup, cleanup, ad‑hoc
SQL, certificate generation, and activity/aggregation helpers.
* Added expanded test utilities for replication, DB connections and test
data seeding to improve integration-test reliability.
* **Chores**
* No production behavior or user-facing APIs were changed.
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[](https://app.coderabbit.ai/change-stack/fleetdm/fleet/pull/45406)
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