[Controls] Fix flaky time slider test (#169553)

Closes https://github.com/elastic/kibana/issues/169404

## Summary

The `Pin start` tooltip has a tendency to get in the way of the
`timeSlider-nextTimeWindow` button:


![image](dc4855ab-6c59-4003-83f1-de778f97b0f8)

To prevent this, I've made it so that the popover is closed **before**
the `next` button is pressed.


### [Flaky Test
Runner](https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/3681)


![image](d339d256-dced-4bbd-b287-e04cd1268835)

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

test/functional/apps/dashboard_elements/controls/common/time_slider.ts

@@ -106,7 +106,6 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
         const valueAfter = await dashboardControls.getTimeSliceFromTimeSlider();
         expect(valueBefore).to.not.equal(valueAfter);
 
-        await dashboardControls.closeTimeSliderPopover();
         await dashboard.clickCancelOutOfEditMode();
         const valueNow = await dashboardControls.getTimeSliceFromTimeSlider();
         expect(valueNow).to.equal(valueBefore);

test/functional/page_objects/dashboard_page_controls.ts

@@ -721,6 +721,7 @@ export class DashboardPageControls extends FtrService {
 
   // Time slider functions
   public async gotoNextTimeSlice() {
+    await this.closeTimeSliderPopover(); // prevents the pin tooltip from getting in the way
     await this.testSubjects.click('timeSlider-nextTimeWindow');
   }