Bug 1494796 - wait for paintflash requests to settle r=jdescottes

Differential Revision: https://phabricator.services.mozilla.com/D32721

--HG--
extra : moz-landing-system : lando

devtools/client/shared/test/browser_telemetry_button_paintflashing.js

@@ -35,6 +35,7 @@ async function testButton(toolbox) {
   ok(button, "Captain, we have the button");
 
   await delayedClicks(toolbox, button, 4);
+  await toolbox.target.client.waitForRequestsToSettle();
   checkResults();
 }