Speed up XPath execution

JSON.stringify calls in two nested cycles are very resource-intensive.
However, the returned value of these calls is the same during loops.
It seems they could be moved outside the loops safely.

Fixes #1473.

lib/jsdom/level3/xpath.js

@@ -600,9 +600,10 @@ module.exports = core => {
     finalize: function() {
       if (null == this.nextPos) return this;
       console.assert(0 === this.nextPos.length);
+      var lastsJSON = JSON.stringify(this.lasts);
       for (var i = 0; i < this.lasts.length; ++i) {
         for (var j = 0; j < this.lasts[i].length; ++j) {
-          console.assert(null != this.lasts[i][j], i + ',' + j + ':' + JSON.stringify(this.lasts));
+          console.assert(null != this.lasts[i][j], i + ',' + j + ':' + lastsJSON);
         }
       }
       this.pushSeries = this.popSeries = this.addNode = function() {