Wait for previous pdfDocument(s) to be destroyed before running the next reference test

Refactors `Driver._cleanup` to return a `Promise` which is resolved once all opened documents have been destroyed.
This is then used in `Driver._nextTask` to ensure that we wait for everything to be cleaned up, such that the tests run sequentially.

test/driver.js

@@ -334,66 +334,67 @@ var Driver = (function DriverClosure() { // eslint-disable-line no-unused-vars
       }, this.delay);
     },
 
-    _nextTask: function Driver_nextTask() {
-      var self = this;
-      var failure = '';
-
-      this._cleanup();
+    _nextTask() {
+      let failure = '';
 
+      this._cleanup().then(() => {
         if (this.currentTask === this.manifest.length) {
           this._done();
           return;
         }
-      var task = this.manifest[this.currentTask];
+        let task = this.manifest[this.currentTask];
         task.round = 0;
         task.pageNum = task.firstPage || 1;
         task.stats = { times: [] };
 
         this._log('Loading file "' + task.file + '"\n');
 
-      var absoluteUrl = new URL(task.file, window.location).href;
+        let absoluteUrl = new URL(task.file, window.location).href;
         PDFJS.disableRange = task.disableRange;
         PDFJS.disableAutoFetch = !task.enableAutoFetch;
         try {
           PDFJS.getDocument({
             url: absoluteUrl,
-          password: task.password
-        }).then(function(doc) {
+            password: task.password,
+          }).then((doc) => {
             task.pdfDoc = doc;
-          self._nextPage(task, failure);
-        }, function(e) {
-          failure = 'Loading PDF document: ' + e;
-          self._nextPage(task, failure);
+            this._nextPage(task, failure);
+          }, (err) => {
+            failure = 'Loading PDF document: ' + err;
+            this._nextPage(task, failure);
           });
           return;
         } catch (e) {
           failure = 'Loading PDF document: ' + this._exceptionToString(e);
         }
         this._nextPage(task, failure);
+      });
     },
 
-    _cleanup: function Driver_cleanup() {
+    _cleanup() {
       // Clear out all the stylesheets since a new one is created for each font.
       while (document.styleSheets.length > 0) {
-        var styleSheet = document.styleSheets[0];
+        let styleSheet = document.styleSheets[0];
         while (styleSheet.cssRules.length > 0) {
           styleSheet.deleteRule(0);
         }
-        var ownerNode = styleSheet.ownerNode;
+        let ownerNode = styleSheet.ownerNode;
         ownerNode.parentNode.removeChild(ownerNode);
       }
-      var body = document.body;
+      let body = document.body;
       while (body.lastChild !== this.end) {
         body.removeChild(body.lastChild);
       }
 
+      let destroyedPromises = [];
       // Wipe out the link to the pdfdoc so it can be GC'ed.
-      for (var i = 0; i < this.manifest.length; i++) {
+      for (let i = 0; i < this.manifest.length; i++) {
         if (this.manifest[i].pdfDoc) {
-          this.manifest[i].pdfDoc.destroy();
+          destroyedPromises.push(this.manifest[i].pdfDoc.destroy());
           delete this.manifest[i].pdfDoc;
         }
       }
+      return Promise.all(destroyedPromises);
     },
 
     _exceptionToString: function Driver_exceptionToString(e) {