Add e2e test for pages, chiefly for publishing pages

[tofumatt]

We had a pretty major bug ship in 3.6.0 (#9054) because we don't have any tests that check for Gutenberg working for pages/in a non-post context.

We should have e2e tests that check for the editor functioning on a New Page as well as New Post screen, and check to make sure users can save new/existing pages with Gutenberg. Given we have similar tests already for posts these should give us plenty of prior art to work from.

Of course, I am wearing the dunce hat for ignoring that issue when it happened, but still: we should have tests for it so it never does 😉

[tofumatt]

@nosolosw beat me to it! 😅