ZTS: Allow longer run time for zdb_args_pos

The zdb_args_pos test may take slightly longer than 600 seconds to run
on some of the CI builders. To prevent this from causing failures allow
up to 1200 seconds for tests in this group.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #15826