pgsql: Make new regression indpendent of max_parallel_workers_per_gath

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0976c4ddd4c98d64b2f3140ee96b92367b763a44

Modified Files
--------------
src/test/regress/expected/partition_aggregate.out | 5 ++++-
src/test/regress/sql/partition_aggregate.sql      | 6 +++++-
2 files changed, 9 insertions(+), 2 deletions(-)