Migrate test harness off deprecated XPCOM interfaces

[1ec5]

The in-application test harness uses a variety of XPCOM interfaces that have been removed:

• nsIAtomService, which was removed for bug 1,393,692, to format table cells: https://github.com/1ec5/avim/blob/9a84083b5f3120f9102ab9b6cc46234b32ed17c2/content/test/tester.js#L272-L274

• nsIFilePicker.show(), removed in favor of nsIFilePicker.open(): https://github.com/1ec5/avim/blob/9a84083b5f3120f9102ab9b6cc46234b32ed17c2/content/test/tester.js#L75-L81

• nsILocalFile, which was removed in favor of nsIFile: https://github.com/1ec5/avim/blob/9a84083b5f3120f9102ab9b6cc46234b32ed17c2/content/test/tester.js#L90