Add test for concurrent access needed to be inside block
Add test for concurrent access needed to be inside block

* Why was this change necessary?

I wanted to check if concurrent access worked correctly and make sure it
will still work if there is a refactor.

* How does it address the problem?

It adds a test for this scenario.

* Are there any side effects?

This was already documented in GitHub but not tested:

https://github.com/thoughtbot/climate_control/issues/32#issuecomment-800713686

thoughtbot/climate_control