We don't want to use the optimizer for planning queries in SQL, pl/pgSQL
etc. functions when that is done on the segments.

ORCA excels in complex queries, most of which will access distributed
tables. We can't run such queries from the segments slices anyway
because they require dispatching a query within another - which is not
allowed in GPDB. Note that this restriction also applies to non-QD
master slices.  Furthermore, ORCA doesn't currently support pl/*
statements (relevant when they are planned on the segments).

For these reasons, restrict to using ORCA on the master QD processes
only.

Also revert commit d79a2c7f ("Fix pipeline failures caused by 0dfd0ebc.")
and separate out gporca fault injector tests in newly added
gporca_faults.sql so that the rest can run in a parallel group.
Signed-off-by: NJesse Zhang <sbjesse@gmail.com>