Gutenberg: Add E2E test to smoke test block invalidation error on default templates

[gwwar]

Let's add some tests around FSE page templates loading correctly after a new site is created.

I'd recommend one canary for the homepage, and other full suites on the remaining templates.

[ianstewart]

While the following screen exists in signup …

We'll want e2e tests for at least the three different homepages. (Blog, Business, Professional.)

[gwwar]

For folks who've never added E2E tests before see also PCYsg-o8X-p2

[gwwar]

We landed #38397 butI think a canary would still be worthwhile.

[mmtr]

We have now a bunch of homepage layouts:

Not sure which one we should pick for the e2e test, but maybe the current theme template could be a good candidate in order to have the e2e more maintainable and less prone to failures when changing the SPT flow.

[mmtr]

Actually, disregard my previous comment. After reading twice the issue summary, I realized that we want to check the default homepage created after creating a new site. There is no template selector on that flow, so I guess that the test could verify the homepage can be loaded in the block editor without any block validation error.

[mmtr]

Added an extra step in #38507 to the basic sign up e2e canary test that verifies there are no errors when editing the homepage.