pgsql: Run select_parallel test by itself

Run select_parallel test by itself

Remove the plpgsql wrapping that hides the context.  So now the test
will fail if the work doesn't actually happen in a parallel worker.  Run
the test in its own test group to ensure it won't run out of resources
for that.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/f9472d72561d285e8c138f3e1276f3110f55e515

Modified Files
--------------
src/test/regress/expected/select_parallel.out | 14 ++++----------
src/test/regress/parallel_schedule            |  5 ++++-
src/test/regress/sql/select_parallel.sql      | 10 ++--------
3 files changed, 10 insertions(+), 19 deletions(-)