campaigns: speed up tests by splitting up integration tests into smaller unit tests

Created by: mrnugget

Our tests in ./enterprise/internal/campaigns have gotten much slower over the past few months, mainly because we've added more tests and those tests rely on the database.

But now, with the reconciler being split into the reconciler, determinePlan, and executor we can, for example, split up the reconciler tests and create smaller tests that don't rely on the database as much.

Since this work can be endless, this should be timeboxed.