Explain how to use the testing backdoor outside of Rails routes

[pangolingo]

Some projects may want to use the backdoor, but may not be using Rails routes in their system tests. One example is an app that hosts a React frontend, but still uses Clearance cookies for auth. A system test with Capybara may visit a React route like this: visit "/payment_method".

The existing documentation didn't show how to use the backdoor with this kind of route.