Changes

tests/db_upgrade: Use log stderr blocking-io

When --db-check is used, osmo-hlr actually becomes a synchronous
non-interactive program instead of a daemon.
In that case, we want to use blocking-io since it doesn't use the event
loop, so we want to flush all logging synchrnously before exiting.

Change-Id: Ic81eacbcc0432b81debc52c49fddc668c7a062b3