tests: Add test for #722696 (icon size request issue)

https://bugzilla.gnome.org/show_bug.cgi?id=722696
Respect 'prefer_bigger' even if icon size is not requested

If gupnp_device_info_get_icon_url() was called without a specific size
request, it returned the last icon in the list.

This patch makes the function return in those circumstances either the
largest icon or the smallest icon, based on 'prefer_bigger' argument
value.

https://bugzilla.gnome.org/show_bug.cgi?id=722696