test(e2e): enable async clipboard testing pref in Firefox (#39838)

When `navigator.clipboard.readText()` runs before the page's copy has
landed, Firefox shows its "Paste" prompt, which cannot be interacted
with when headless. Avoid that and and use
`dom.events.testing.asyncClipboard` which is a pref meant for testing
that disabes this prompt.

Fixes: https://github.com/go-gitea/gitea/actions/runs/39113819632/job/117635434106
Signed-off-by: silverwind <me@silverwind.io>

playwright.config.ts

@@ -34,6 +34,11 @@ export default defineConfig({
       name: 'firefox',
       use: {
         ...devices['Desktop Firefox'],
+        launchOptions: {
+          firefoxUserPrefs: {
+            'dom.events.testing.asyncClipboard': true, // disable paste prompt
+          },
+        },
       },
     },
   ],