Fix specs so they pass when run all together or in isolation.

- Remove `Client.delete_all` form the `describe` block. This gets
  executed when the spec file is being loaded, which is long before
  the examples in the group are executed, and many specs can run
  in between these two times, so it doesn’t accomplish anything
  useful.
- Truncate the DB in an `after(:all)` hook in the contexts that
  use a `before(:all)` hook. Since that runs outside a transaction
  we need to clean up manually.