test: deflake tween mouse test (#1515)

When mojo is enabled for input events, we sometimes get an extra mouse move after the page is created. Moving the mouse ourselves makes the test consistent. The same behavior also caused DevTools dispatchMouseEvent to become flaky, but that will be fixed with: https://chromium-review.googlesource.com/c/chromium/src/+/804357

test/test.js

@@ -2207,16 +2207,15 @@ describe('Page', function() {
       await page.click('a');
     }));
     it('should tween mouse movement', SX(async function() {
+      await page.mouse.move(100, 100);
       await page.evaluate(() => {
         window.result = [];
         document.addEventListener('mousemove', event => {
           window.result.push([event.clientX, event.clientY]);
         });
       });
-      await page.mouse.move(100, 100);
       await page.mouse.move(200, 300, {steps: 5});
       expect(await page.evaluate('result')).toEqual([
-        [100, 100],
         [120, 140],
         [140, 180],
         [160, 220],