Fix flaky playwright Theia Main Menu test

We are observing cases where the `.getByRole('button', { name: 'OK' })` resolved to multiple elements (e.g. [here](https://github.com/eclipse-theia/theia/actions/runs/10025491100)).

With this change we limit the selection to the dialog shell and hopefully remove this ambiguity.

Contributed on behalf of STMicroelectronics.

Change-Id: I5ee5b3fc6a683f68da99e219b32784c3f5d4bfb7

Author: planger

examples/playwright/src/tests/theia-main-menu.test.ts

@@ -97,7 +97,7 @@ test.describe('Theia Main Menu', () => {
         await (await menuBar.openMenu('Help')).clickMenuItem('About');
         const aboutDialog = new TheiaAboutDialog(app);
         expect(await aboutDialog.isVisible()).toBe(true);
-        await aboutDialog.page.getByRole('button', { name: 'OK' }).click();
+        await aboutDialog.page.locator('#theia-dialog-shell').getByRole('button', { name: 'OK' }).click();
         expect(await aboutDialog.isVisible()).toBe(false);
     });