Random failure with scenario "Delete an event" (take #2)

This is a followup of MDL-67910, where a good number of failures with the "Delete an event" were fixed.

Worth noting that we were not able to 100% kill the randoms and they continued happening here and there, although never twice.

But, it seems that some recent change in the JS in that area (maybe MDL-68409) have caused the failures to be back and often than ever. Only master, they are happening a lot of times, twice (leading to failed runs).

Some links:

• https://ci.moodle.org/job/W.03.03%20-%20Behat%20-%20Chrome%20+%20MariaDB%20(lowest)/611/
• https://ci.moodle.org/view/B%20-%20master/job/W.03.04%20-%20Behat%20-%20Chrome%20+%20MSSQL/611/

(curiously, I've not been able to find any postgres run with that failure, maybe that's just a coincidence).

001 Scenario: Delete an event                # /var/www/html/calendar/tests/behat/calendar.feature:102

      And I click on "Delete event" "button" # /var/www/html/calendar/tests/behat/calendar.feature:112

        Button matching locator "Delete event" not found. (Behat\Mink\Exception\ElementNotFoundException)