Add a unit-test that attempts to fetch a non-existent named destination

Doing this helped uncover an issue with the `getDestination` implementation.
Currently if a named destination doesn't exist, the method (in `obj.js`) may return `undefined` which leads to the promise being stuck in a pending state.
*Note:* returning `null` for this case is consistent with other methods, e.g. `getOutline` and `getAttachments`.

src/core/obj.js

@@ -485,7 +485,7 @@ var Catalog = (function CatalogClosure() {
       }
 
       var xref = this.xref;
-      var dest, nameTreeRef, nameDictionaryRef;
+      var dest = null, nameTreeRef, nameDictionaryRef;
       var obj = this.catDict.get('Names');
       if (obj && obj.has('Dests')) {
         nameTreeRef = obj.getRaw('Dests');