Add e2e tests for entity reference tgs in page data form

Tested this manually and works for me.

Reviewed and run the tests locally and all good. 🚢

Test-only changes — I feel comfortable committing this without @bnjmnm looking at this then :D

@larowlan suggested reverting this due to intermittent reliability here . I opted to create a new branch instead 3517859-make-test-more-stable so we can build off what is there + there's a comment pointing out how the wait() seems to fix it. There will likely be a @todo pointing to this issue in 0.x shortly.

I _think_ I've found the issue and a way around it by making use of cy.tick to ensure timeouts are triggered
Passed 5 times straight locally, let's see what CI says

This only needs approval from @bnjmnm 😊

And if this works, then this likely is a test hardening strategy we'll need in many more places?