As well as providing a unit test which matches other persistence
provider tests (round-tripping a workflow instance), two other tests
are included here:
* When using the default DI registration extension method, this sets
up pooling of DB contexts and so a resolved context will come from
the pool.
* When using the non-pooled DI registration extension method, this
sets up EF without pooling, which is more useful in a multi-tenanted
environment.
There are also some other changes in this commit corresponding to
improving the tests. Mainly de-duplicating logic & improving names.
