resource-test: Use wl_seat instead of wl_display for testing

We're creating resources with versions up to 4.  wl_display isn't version 4,
so this is technically verifying that we can do something we shouldn't.

wl_seat already has versions this high, so switch to that.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>