pgsql: Add a test case for a316a3bc

Add a test case for a316a3bc

a316a3bc fixed the code in build_simpl_rel() that propagates
RelOptInfo.userid from parent to child rels so that it works
correctly for the child rels of a UNION ALL subquery rel, though
no tests were added in that commit. So do so here.

As noted in the discussion, coming up with a test case in the core
regression suite for this fix has turned out to be tricky, so the
test case is added to the postgres_fdw's suite instead.
postgresGetForeignRelSize()'s use of user mapping for the user
specified in RelOptInfo.userid makes it relatively easier to craft
a test case around.

Discussion: https://postgr.es/m/CA%2BHiwqH91GaFNXcXbLAM9L%3DzBwUmSyv699Mtv3i1_xtk9Xec_A%40mail.gmail.com
Backpatch-through: 16

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/054ff3b33a85d62b4d486dabb5e5718399822a18

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 42 ++++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 23 ++++++++++++++
2 files changed, 65 insertions(+)