Convert the state-pension-age flow tests
Convert the state-pension-age flow tests

- converts the state-pension-age tests to use the node based testing
approach introduced in [this ADR](https://github.com/alphagov/smart-answers/blob/main/docs/arch/003-testing-flows.md)

- Some code was found to be unreachable. This has been described using comments in the test. The redundant code and test comments will be removed in upcoming commits.

alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers