Add e2e test to cover Nav block front end fallbacks

[getdave]

In #36740 we added functionality to provide a fallback for the Nav block.

Let's improve test coverage around that by adding e2e tests.

I left a few suggestions related to naming, but otherwise this looks good to me! If you added a test or two, that would be absolutely stellar. Otherwise, 🚢

Originally posted by @adamziel in #36740 (review)

[t-hamano]

It's been 4 years since this issue was submitted, so I'd like to know if this issue is still valid.

If so, I'd be grateful if you could tell us what specific e2e testing is required.

cc @himanshupathak95