[Detection Engine][FTR] Unskip alerts migration test

[yctercero]

Tracking skipped test - https://github.com/elastic/kibana/blob/v8.13.0/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/trial_license_complete_tier/migrations/finalize_alerts_migrations.ts#L193

Per Ryland's comment:

[...] it's been skipped since #85690. I don't recall exactly why it was skipped, and it doesn't look like I left any clues for us 😢 .

However, I was surprised to see the build was green after unskipping this. It looks to be a false positive, as the actual assertions of this test are commented out below (and most obviously, no getMigration helper currently exists).

We'll need to determine whether to unskip or delete.

[elasticmachine]

Pinging @elastic/security-detection-engine (Team:Detection Engine)