tests: Services can be defined by name, host, hostgroup

Apparently, name was broken up until recently - see 933729a - but now it
seems to work again. Jay.

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>
objects: Improve errors for missing service args

"Error: Service description, host name, or check command is NULL", you
say? Well, your mom, sister, or aunt is NULL!

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>