Set synchronous_commit=on in test_setup.sql.

Set synchronous_commit=on in test_setup.sql.

Starting in cc50080a82 create_index test fails when run with
synchronous_commit=off. synchronous_commit=off delays when hint bits may be
set. Some plans change depending on the number of all-visible pages, which in
turn can be influenced by the delayed hint bits.

Force synchronous_commit to `on` in test_setup.sql. Not very satisfying, but
there's no obvious alternative.

Reported-By: Aleksander Alekseev <aleksander@timescale.com>
Author: Andres Freund <andres@anarazel.de>
Author: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://www.postgresql.org/message-id/flat/CAJ7c6TPJNof1Q+vJsy3QebgbPgXdu2ErPvYkBdhD6_Ckv5EZRg@mail.gmail.com