Fix gr-tooltip test

We have to use flush after updating the element so to fix the test,
we simply do a flush in setup().

This seems to be an issue observed in Chrome v69 (used for
stable branches), whilst it works in the latest chrome releases.

Change-Id: I34039db29f18740ee10a591ea94f3723de4c1bde

polygerrit-ui/app/elements/shared/gr-tooltip/gr-tooltip_test.js

diff --git a/polygerrit-ui/app/elements/shared/gr-tooltip/gr-tooltip_test.js b/polygerrit-ui/app/elements/shared/gr-tooltip/gr-tooltip_test.js
index b5f068c..65a442b 100644
--- a/polygerrit-ui/app/elements/shared/gr-tooltip/gr-tooltip_test.js
+++ b/polygerrit-ui/app/elements/shared/gr-tooltip/gr-tooltip_test.js
@@ -26,8 +26,9 @@
 
 suite('gr-tooltip tests', () => {
   let element;
-  setup(() => {
+  setup(async () => {
     element = basicFixture.instantiate();
+    await flush();
   });
 
   test('max-width is respected if set', () => {