Add tests to guard against reconciliation errors (themes)

[ockham]

Calypso's /themes and /theme routes, when visited by a logged-out user, make use of Server-Side Rendering (SSR) for SEO and performance benefits. The latter is hampered by reconciliation errors that tend to sneak in easily, see e.g. https://github.com/Automattic/wp-calypso/issues/13376. It'd be nice to guard against this kind of error with an e2e test.

[alisterscott]

I believe @rachelmcr has added a sign up test for a premium theme purchase from a signed out /themes start, are we able to close this?

[ockham]

Hmm, this isn't really about any flows (such as purchase etc) but rather about checking for the presence of a console error complaining about a mismatch between server-rendered and client-rendered layouts, right when landing on an SSR'd page.

Automattic/wp-calypso#13376 illustrates what these normally look like:

To repro:

• Run Calypso locally (error is hidden in production)
• In an incognito window, land at http://calypso.localhost:3000/themes
• Check the console:

(Error messages have changed per React 16 IIRC.)

[alisterscott]

Thanks @ockham for the context

I'll keep this open to see what we can add for this