[#65271738] Clarify intention of unit tests

Previously, each unit test tested several expectations. This refactor uses contexts to clarify with separate tests what is expected from each scenario.

Re-writing the tests in this way makes two outstanding bugs even more obvious:

 - lack of dependency injection in the Nat Service requires repeated set-up for each context
 - config is empty if `update_required?` has not been called

There are stories already outstanding to deal with these issues.
Merge pull request #3 from alphagov/improve-update-unit-tests

[#65271738] Clarify intention of unit tests