Make test_all_trigger a virtual job, thus no agent allocation

[todi1856]

[u-pr]

💡 Harness Review

The change cleanly converts the aggregate test trigger into a dependency-only virtual job. I examined comparable Yamato jobs, dependency propagation, artifact producers and downstream consumers, and found no actionable issues.

_{Reviewed commit e0f23e2}

_{🤖 Helpful? 👍/👎}