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