Make new regression indpendent of max_parallel_workers_per_gather.

Make new regression  indpendent of max_parallel_workers_per_gather.

The tests in e2f1eb0ee30d1 ("Implement partition-wise
grouping/aggregation.") weren't independent of the server's
max_parallel_workers_per_gather setting.  I (Andres) find it useful to
locally run with that disabled, and the aforementioned patch broke
this.

Author: Jeevan Chalke
Discussion:
    https://postgr.es/m/20180322210703.qmga3vsxqmiiypci@alap3.anarazel.de
    https://postgr.es/m/CAM2+6=UNWGKTgh9aOn4=SQ72HfFzbVFseh9=5N54bD6KB+D9OQ@mail.gmail.com