pgsql: pg_plan_advice: Fix multiple copy-and-paste-errors in test case.

pg_plan_advice: Fix multiple copy-and-paste-errors in test case.

The second half of this file is meant to test feedback, not
generated advice, and is meant to use the statements that it
prepares, not leftover prepared statements from earlier in the
file.

These mistakes resulted in failures under debug_discard_caches = 1,
because re-executing pt2 instead of executing pt4 for the first
time resulted in different output depending on whether the query
was replanned.

Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> (per BF member avocet)

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b335fe56f35506348a567648e3a7ac1b3ac0baf3

Modified Files
--------------
contrib/pg_plan_advice/expected/prepared.out | 14 ++++++++------
contrib/pg_plan_advice/sql/prepared.sql | 6 +++---
2 files changed, 11 insertions(+), 9 deletions(-)