Bug 1713130 - Fix browser_eventDialogSize.js to work in isolation. r=darktrojan

The test was relying on the calendar tab being still open from the previous test.

Differential Revision: https://phabricator.services.mozilla.com/D132242

calendar/test/browser/eventDialog/browser_eventDialogSize.js

@@ -17,6 +17,7 @@ registerCleanupFunction(() => {
 });
 
 add_task(async function testEventDialog() {
+  await CalendarTestUtils.openCalendarTab(window);
   let { dialogWindow, iframeWindow } = await CalendarTestUtils.editNewEvent(window);
   await checkLargeEnough(dialogWindow, iframeWindow);