Playwright: theme-activate spec has clash

### Details

The spec `wp-theme__details-preview-activate.js` in the Playwright suite has an isolation problem.

If there are two parallel instances Playwright e2e tests running at the same time:

- execution A logs in and begins the test steps.
- execution B logs in and begins the test steps, just a few seconds after execution A.
- the test account has Twenty Twenty-One active, wants to activate Twenty-Twenty.

Execution A would get there first; it searches for and activates Twenty Twenty-One.
Execution B does not know this, as it is looking at the Themes gallery perhaps half a second after execution A. It clicks into Twenty Twenty-One expecting to find an 'Activate' button but instead it finds a `Customize theme` button.

This causes the step to register as a failure.

### Checklist

Possible ideas
- [x] discontinue activate portion of the test?
- [ ] create a new site for each instance of this test?

### Related

_No response_

Start with the Playwright spec wp-theme__details-preview-activate.js and run the relevant test with parallel instances. Check how concurrent executions share the test account and confirm the chosen change prevents one execution from seeing a theme already activated by another; done means the spec no longer fails from the Activate/Customize theme clash.

Written by the indexing model from the issue text.