cc @kevin940726

I think we might have a misunderstanding here. I didn't mean to rewrite testing utilities in Core, we should be able to continue using utilities from gutenberg, but rather to rewrite them in gutenberg and try to make them faster.

Take isThemeInstalled for example. You did a great job here re-implementing it using REST API for maximum performance. While the original code in gutenberg still requires several navigations (visitAdminPage or page.goto) and element selections (page.waitForSelector or page.$) to achieve the same thing.

I think we should instead re-implement it using the approach you do here in gutenberg. However, that would also mean that we'll be losing the coverage of that in our tests. The solution is to add back those here in Core instead, but we don't need to make it a utility, just a simple test.

Basically, swap the code in gutenberg with Core's 😅 . Does it make sense to you? I'll c.c. @tellthemachines for better explanations or other feedbacks 😆 .