pgsql: Fix regression test for force_parallel_mode=on.

Fix regression test for force_parallel_mode=on.

Commit 14a254fb52423c57059851abafbd1247261f7f03 managed not to
exercise the code it was intended to test, and the comment explaining
why no "parallel worker" line showed up in the context wasn't right.

Amit Kapila, tweaked by me per Amit's analysis.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/12f862099d25fc70b412d56f50dcabebff8db44a

Modified Files
--------------
src/test/regress/expected/select_parallel.out | 13 +++++++------
src/test/regress/sql/select_parallel.sql      |  8 +++++---
2 files changed, 12 insertions(+), 9 deletions(-)