Ignore local test theme folders created by wp-env

[WunderBart]

What?

Add test themes folders created by wp-env to .gitignore so they're ignored by linter.

Why?

Because wp-env creates these folders with very restrictive permissions and they have to be removed by a root user to make the commit linter pass.

Testing Instructions

• Run npx wp-env start --update - you should see twentytwentyone and twentytwentythree be downloaded and installed as per this config.
• Confirm that the above theme folders have been created inside /test/gutenberg-test-themes/ folder (they should be empty).
• Make any change to any file and try to commit it - you should be able to do this without the aforementioned issue on this branch and not on trunk.

[tyxla]

I just encountered this - great timing!

Thanks for fixing 🚀

[github-actions]

Flaky tests detected in b7bc7a7.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/5681175779
📝 Reported issues:

• [Flaky Test] Animation selector should NOT appear if Behavior is None #52560 in /test/e2e/specs/editor/blocks/image.spec.js
• [Flaky Test] Animation selector should appear if Behavior is Lightbox #52371 in /test/e2e/specs/editor/blocks/image.spec.js
• [Flaky Test] lightbox should be overriden when link is configured for image #52348 in /test/e2e/specs/editor/blocks/image.spec.js
• [Flaky Test] should toggle "lightbox" in saved attributes #52350 in /test/e2e/specs/editor/blocks/image.spec.js
• [Flaky Test] zoom animation #52381 in /test/e2e/specs/editor/blocks/image.spec.js

[mirka]

Works for me as well. Thank you!