Force synchronous commit to be enabled for all test_decoding tests.

Force synchronous commit to be enabled for all test_decoding tests.

Without that the tests fail when forced to be run against a cluster
with synchronous_commit = off (as the WAL might not yet be flushed to
disk by the point logical decoding gets called, and thus the expected
output breaks). Most tests already do that, add it to a few newer tests.

Author: Andres Freund