Provide a NativeImage icon in the notification tests and then inspect
the DBus message payload's `image_data` hint to see if it's correct.
This adds test coverage for LibnotifyNotification::Show() and for
GdkPixbufFromSkBitmap().
Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Charles Kerr <charles@charleskerr.com>
