Introduce our own window property to distinguish between test windows from normal Brackets window.

[RaymondLim]

This fixes issue #3219.

[jasonsanjose]

I tested the fix on mac and it works, I don't see any test windows remaining open. However, the docs in ShellAPI aren't very clear. When executeCommand returns false, what/who is that signaling?

I understand that no commands are run if there are any modal dialogs open. I also understand that for for Commands.FILE_CLOSE_WINDOW we swallow this command and return true when any dialogs are open in order to prevent losing unsaved changes, see #3152. I don't see why return false here allows the test window to close. Maybe I'm mis-reading something.

[RaymondLim]

The code that returning false when a modal dialog is up is for issue #2616, telling native shell that js command is not handling it.

[jasonsanjose]

Merging.