Allow mocking the local hregion as an htable.

The issue is it's not possible to wrap the existing hregion mock in a `HRegionAsHTable` adapter since there's no easy way to make it satisfy the needed java interface.

So mock around it instead, by fixing a single `ExecutionContext` instance and using real `Relation` objects wrapping doubles.

Note we stub the execution context out on the `Runner` by hijacking the thread local: the runner itself can't be stubbed since it is frozen on construction.