Fix broken Playwright tests

[brandonpayton]

Motivation for the change, related issues

Some Playwright tests were broken by last-minute changes to #1731.

Fixes #1818

Implementation details

• Fix selection of "edit settings" and "save settings" buttons to select "Create a similar Playground" and "Create" buttons instead.
• Change playground viewport selection to account for there being multiple possible viewports now. We were using an ID, but now we're using a CSS class.
• Some expect() calls were failing, at least locally, and that was fixed by awaiting them. They need to be async.

Testing Instructions (or ideally a Blueprint)

• CI

[brandonpayton]

The Blueprint tests are still failing, and I'm looking into it.

[brandonpayton]

Actually, the Blueprint tests were fine but anything related to WP multisite fails when the local server has a non-standard port (5400 in our case).

[brandonpayton]

@bgrgicak, why do we opt out of parallel tests on CI, and should we continue opting out of parallel tests when running locally?

I enabled more workers to speed local testing, but we can change it back if needed.

[bgrgicak]

They were occasionally failing locally and this made them more robust.
I just went with disabling parallel tests to get it out quicker. We can test it now with multiple workers and see how well it works.

[brandonpayton]

Let's merge this so CI tests complete again and make any adjustments in follow-up PRs.