Playwright Utils: Use 'set' to disable the Styles welcome guide

[Mamaduka]

What?

Resolves #49942

PR updates the visitSiteEditor utility method to use the set action to disable the Styles welcome guide.

Why?

The toggle action doesn't accept value as an argument and can sometimes enable the welcome guide modal.

Testing Instructions

1. Add a new test to the "Push to Global Styles button". See the code below.
2. Running the test on the trunk will fail, but will pass on this branch.

npm run test:e2e:playwright -- test/e2e/specs/site-editor/push-to-global-styles.spec.js

test( 'should apply condition', async ( { page } ) => {
        // Turn off the welcome guide.
        await page.evaluate( () => {
            window.wp.data
                .dispatch( 'core/preferences' )
                .set( 'core/edit-site', 'welcomeGuideStyles', false );
        } );
} );

[github-actions]

Flaky tests detected in b031284.
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/5049332804
📝 Reported issues:

• [Flaky Test] should delete the template after saving the reverted template #50783 in /test/e2e/specs/site-editor/template-revert.spec.js
• [Flaky Test] should show the original content after revert #47897 in /test/e2e/specs/site-editor/template-revert.spec.js

[talldan]

Makes sense. It's weird that it's not set already, but looks like it was my mistake 😄