test: include actual value in assertion message

PR-URL: nodejs/node#15935
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: mattcan

test/addons-napi/test_dataview/test.js

@@ -11,4 +11,4 @@ const template = Reflect.construct(DataView, [buffer]);
 
 const theDataview = test_dataview.CreateDataView(template);
 assert.ok(theDataview instanceof DataView,
-          'The new variable should be of type Dataview');
+          `Expect ${theDataview} to be a DataView`);