Add testing for ErrorPage & LoadingOverlay

[seanmturley]

Description

As far as I can see, testing the ErrorPage component requires testing and rendering the full App with specific URLs that will generate different types of errors. Unfortunately, App renders a router (the router, in fact), so when it is rendered with setupWithRouting or the now removed setupWithUrl testing utility functions the following error occurs:

"You cannot render a inside another . You should never have more than one in your app."

The same issue is true for testing the LoadingOverlay, which is rendered by the MainLayout.

Questions

• How can ErrorPage be tested?
• Is there a way around the above error when rendering App for tests, or another approach entirely?
• Realising the same issue applies to the LoadingOverlay raises the question - can MainLayout be what is rendered/tested to check both the ErrorPage and LoadingOverlay, since it wraps the rest of the app? Answer - it's possible to do this, but the full routing including Outlets and children need to be supplied to the setupWithRouting testing utility function.
• Following from the previous point, the next problem that arises is that the API call doesn't work from within the tests, so that needs to be mocked.

[seanmturley]

Added notes to the OP to describe the options of testing at the layout level (and rendering Outlets), plus the need to mock API requests.

This is quite involved, and more research is required to determine the best way forward.

[seanmturley]

I've abandoned local changes for now as nothing much was achieved in implementing the tests, and I want to move onto new features. This task will be set aside to revisit later.