Normalize preload test by resetting preferences before the test starts

This test kept failing for me, and I isolated it to if a test before it had run await admin.visitSiteEditor(), then the preload test would fail because the user had previously visited the site editor. By resetting preferences before the test runs, it allows a cleaner enviroment and the test has consistent results.

test/e2e/specs/site-editor/preload.spec.js

@@ -6,6 +6,7 @@ const { test, expect } = require( '@wordpress/e2e-test-utils-playwright' );
 test.describe( 'Preload', () => {
 	test.beforeAll( async ( { requestUtils } ) => {
 		await requestUtils.activateTheme( 'emptytheme' );
+		await requestUtils.resetPreferences();
 	} );
 
 	test.afterAll( async ( { requestUtils } ) => {