tests: Don't fail due to high default timeout in postmaster/003_start_stop

tests: Don't fail due to high default timeout in postmaster/003_start_stop

Some BF animals use very high timeouts due to their slowness. Unfortunately
postmaster/003_start_stop fails if a high timeout is configured, due to
authentication_timeout having a fairly low max.

As this test is reasonably fast, the easiest fix seems to be to cap the
timeout to 600.

Per buildfarm animal skink.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/ggflhkciwdyotpoie323chu2c2idpjk5qimrn462encwx2io7s@thmcxl7i6dpw