Fix a failing WYSIWYG editor test:
* Set the value of the source text area instead of sending keyboard events because the browser hangs when the text is long and we don't test the typing in the source text area anyway.
* Use a different text area id than the name of the global variable holding the reference to the editor instance because newer verions of Firefox make the DOM elements that have id available as global variables.
(cherry picked from commit 210ca8cf1646c4c210f84bad67f0aa046471148e)