Add documentation on how to mock global components

[ffxsam]

What problem does this feature solve?

When testing components that include globally registered components, the test runner (in this case, Jest) throws warnings asking if the component was registered properly. As far as I can tell, the documentation doesn't explain how to mock global components.

What does the proposed API look like?

N/A

[eddyerburgh]

The option you're looking for is stubs.

Do you have any suggestions for how we can make this clear? Maybe I guide on mocking components and properties?

[ffxsam]

Yes, 100%! I was surprised to not see a guide about stubbing components.

Thanks!

[eddyerburgh]

I'm removing comments from this issue since they devolved into an argument. The plan is to add a guide to the documentation on stubbing components and mocking properties.