test: unflake 'Copy Prompt' tests (#35714)

Author: mxschmitt

tests/playwright-test/reporter-html.spec.ts

@@ -2890,6 +2890,7 @@ for (const useIntermediateMergeReport of [true, false] as const) {
 
       await page.getByRole('link', { name: 'sample' }).click();
       await page.getByRole('button', { name: 'Copy prompt' }).click();
+      await page.waitForFunction(() => navigator.clipboard.readText());
       const prompt = await page.evaluate(() => navigator.clipboard.readText());
       expect(prompt, 'first line').toContain(`Playwright test failed.`);
       expect(prompt, 'contains error').toContain('expect(received).toBe(expected)');
@@ -2930,6 +2931,7 @@ for (const useIntermediateMergeReport of [true, false] as const) {
       await page.context().grantPermissions(['clipboard-read', 'clipboard-write']);
       await page.getByRole('link', { name: 'sample' }).click();
       await page.getByRole('button', { name: 'Copy prompt' }).click();
+      await page.waitForFunction(() => navigator.clipboard.readText());
       const prompt = await page.evaluate(() => navigator.clipboard.readText());
       expect(prompt, 'contains snapshot').toContain('- button "Click me"');
     });

tests/playwright-test/ui-mode-trace.spec.ts

@@ -570,6 +570,7 @@ test('fails', async ({ page }) => {
   await page.context().grantPermissions(['clipboard-read', 'clipboard-write']);
   await page.getByText('Errors', { exact: true }).click();
   await page.locator('.tab-errors').getByRole('button', { name: 'Copy prompt' }).click();
+  await page.waitForFunction(() => navigator.clipboard.readText());
   const prompt = await page.evaluate(() => navigator.clipboard.readText());
   expect(prompt, 'contains error').toContain('expect(received).toBe(expected)');
   expect(prompt.replaceAll('\r\n', '\n'), 'contains test sources').toContain(`