tests: Simplify abstract-interfaces check with a helper

Add a new type_list_find() helper to device-introspect-test.c, to
simplify the code at test_abstract_interfaces().

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170707122215.8819-2-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
