Removes items in the childEls array based on the return value of a supplied test
function. The function is called with a entry in childEls and if the test function
return true, that entry is removed. If false, that entry is kept.

@param {Function} testFn The test function.
function (testFn) {
        var me = this,
            old = me.getChildEls(),
            keepers = (me.childEls = []),
            n, i, cel;

        for (i = 0, n = old.length; i < n; ++i) {
            cel = old[i];
            if (!testFn(cel)) {
                keepers.push(cel);
            }
        }
    }