fix(axe.run): allow returning a Promise in jsdom (#943)

This patch enables `axe.run()` to return a Promise when `window.Promise` is not defined, but there is a Promise implementation in the global scope.

This, while very edge-casey, enables me to run axe in a node process by only importing [`jsdom-global/register`](https://github.com/rstacruz/jsdom-global).

lib/core/public/run.js

@@ -105,7 +105,7 @@ axe.run = function (context, options, callback) {
 	let reject = noop;
 	let resolve = noop;
 
-	if (window.Promise && callback === noop) {
+	if (typeof Promise === 'function' && callback === noop) {
 		p = new Promise(function (_resolve, _reject) {
 			reject = _reject;
 			resolve = _resolve;
@@ -143,4 +143,4 @@ axe.run = function (context, options, callback) {
  	});
 
  	return p;
-};
+};