Instead of appending a script tag with src attribute  (e.g. <script src="/script.js">); this reads the script file and inserts the code within a script tag. This is bec. we cannot know when a dynamically inserted script loads. see https://github.com/tmpvar/jsdom/issues/640#issuecomment-22216965
function _promiseAppendJQuery(obj) {
    const document = obj.document || obj;
    // append to body
    return Promise.resolve()
        .then(() => {
            if (JQUERY_SCRIPT) return JQUERY_SCRIPT;
            return fs.readFile(JQUERY_PATH, 'utf8');
        })
        .then(src => {
            JQUERY_SCRIPT = src;
            const el = document.createElement('script');
            el.type = 'text/javascript';
            el.innerHTML = src;
            el.className = 'jsdom-jquery';
            document.body.appendChild(el);
            return obj;
        });
}