Don't use asynchronous test to check MathMLElement onevent attributes

[fred-wang]

cc @rwlbuis @bkardell

In mathml/relations/html5-tree/clipboard-event-handlers.tentative.html
and mathml/relations/html5-tree/math-global-event-handlers.tentative.html

@bzbarsky suggested we used the following instead:

var callbackCalled = false;
function callback() {
      callbackCalled = true;
}
doStuffThatShouldCallCallback();
assert_true(callbackCalled);

[bzbarsky]

So concretely, in math-global-event-handlers.tentative.html the code could be:

    test(() => {
      const element = document.createElementNS(
        "http://www.w3.org/1998/Math/MathML",
        "math"
      );
      let target = undefined;
      element[name] = (e) => {
        target = e.currentTarget;
      });

      element.dispatchEvent(new Event(withoutOn));
      assert_equals(target, element,
                             "The event must be fired at the <math> element");
    }, `${name}: dispatching an Event at a <math> element must trigger element.${name}`);

in place of the current async_test bit.