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
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master

Details
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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(-)