Add basic swarming UI tests

appengine/swarming/ui/res/imp/common/pageable-data.html

 <!--
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <pageable-data>
 
   The pageable-data element allows for easy paging of data from the swarming
   server. It displays a button to the user to go fetch more data, which will
   be appended to the output, rather than replaced, which is typical in
   elements like iron-ajax.
@@ skipping 137 matching lines @@
           var vals = this.parse(json);
           // !this._cursor means this is our first load and we should empty the
           // array
           if (!this._cursor || !this.output) {
             this.set("output", vals);
           } else {
             this.push("output", ...vals);
           }
           this.set("_cursor", json.cursor || END);
           this.set("busy", false);
+          // for unit testing
+          this.fire("_finishedLoading", {success:true});
         }.bind(this)).catch(function(reason){
           console.log("Reason for failure of request to " + this._url, reason);
           sk.errorMessage("Could not get next set of results. Try reloading the page?", 0);
           this.set("busy", false);
+          // for unit testing
+          this.fire("_finishedLoading", {success:false});
         }.bind(this));
       },
 
       _noMore: function(cursor) {
         return cursor === END;
       }
     });
   })();
   </script>
 </dom-module>