function testCubicBezierClick(opts) {
                    makeTimingFuncUI("cubic-bezier(.42, 0, .58 ,1)");
                    var $item = $(timingFuncEditor[opts.item]);
                    eventAtOffset("click", $item, opts.clickAt);
                    expectArraysToBeEqual(timingFuncEditor._cubicBezierCoords, opts.expected);
                }
Test a mouse down event on the given UI element in a cubic-bezier function.
@param {object} opts The parameters to test:
    item: The (string) name of the member of BezierCurveEditor that
         references the element to test.
    clickAt: An [x, y] array specifying the simulated x/y mouse position as
         an offset of the item's width/height.
    expected: The expected array of values for _cubicBezierCoords.