The installcheck run takes a sizable fraction of test.sh to run. Using
a parallel schedule reduces that noticably.

It's possible that we want to backpatch this at some point, to reduce
buildfarm times, but for now lets just see if this upsets the
buildfarm.

Author: Andres Freund
Discussion: https://postgr.es/m/20181105210039.hh4vvi4vwoq5ba2q@alap3.anarazel.de