Answer the temporal aggregates as MobilityDB answers them

[estebanzimanyi]

GeneratedSurfaceTest runs tCount, tSum, tMinAgg, tMaxAgg and mergeAgg over
three temporal integers, and tAndAgg and tOrAgg over two temporal booleans,
each value in a partition of its own, so the transition runs in every
partition and the combine joins the partial states the shuffle carries
between them. Each answer is the text MobilityDB answers for the same
aggregate over the same values in UTC. tSum over a temporal integer and a
temporal float in two partitions raises the refusal of a mix of types.

Measured. Against JMEOS with the generated aggregates, the libmeos of
MobilityDB 510c3184f6 and the catalog derived from its installed headers,
the suite runs 20 tests, all passing; with the combine of the aggregator
dropped, tCount answers null and the test fails.

Why. The generated surface registers the aggregates, and a partial state
that crosses executors is the one path the scalar tests never take.