Fix test for subplans in force-parallel mode.

Fix test for subplans in force-parallel mode.

We mustn't force parallel mode if the query has any subplans, since
ExecSerializePlan doesn't transmit them to workers.  Testing
top_plan->initPlan is inadequate because (1) there might be initPlans
attached to lower plan nodes, and (2) non-initPlan subplans don't
work either.  There's certainly room for improvement in those
restrictions, but for the moment that's what we've got.

Amit Kapila, per report from Andreas Seltenreich

Discussion: <8737im6pmh.fsf@credativ.de>