replace CommandManager.execute() with DocumentManager.getDocumentForPath() when jump to a new file

Currently `unittests.js` fails when jump to a new file due to CommandManager.execute() failed as no such command in _commands.

So the work around is to use codes from DocumentCommandHandlers.js:doOpen() to handle the case.  `unittests.js` need notify EditorManager for created mockup editor, also when new file is opened, we need get activeEditor instead of early created mockup editor to get new cursor location.