abstract out the addon-page url check

I know this module is deprecated but since I'm still using it I figure I'll offer up my fixes.  I've got another one coming.

This change takes the addon-page url test that's used for hiding the chrome and brings it out into a separate function such that an addon-page which was opened by this addon will also be closed on unload.  In the tests I just removed the final test that only looked at pure addon urls and did a `isTabOpen()` check after each unload of the different urls this supports.