Read a run's entities back from storage through one shared test helper

Forty-four private reload() helpers across forty-two test classes, in five spellings that disagreed with one another and none of which said anything about what came back. ReloadEntityTrait says it once, with a method per runtime entity and an assertion on the result, and findInstance() for the separate question of whether a run is still there at all.

What stays written out inline is a different question: finding which entities match something is a query, and emptying a whole storage cache before a measurement is a statement about the cache rather than a read.

No test changes what it asserts. Test-only: no shipped class is touched.

Issue: https://www.drupal.org/project/orchestra/issues/3622894