This adds coverage for a non-delivery transition from vcl_hit, where the
busy objcore would drop its sole reference in the event of a grace hit.
The lack of coverage was visible in the gcov dashboard:

    669   580   if (busy != NULL) {
    670   0     	(void)HSH_DerefObjCore(wrk, &busy, 0);
    671   0     	VRY_Clear(req);
    672   0     }

There should now be at least one pass inside this block.

Refs #4032