ZTS: Increase zpool_import_parallel_pos import margin

Increase the pool import time allowed by assuming a minimum reduction
to 1/2 instead of 1/3 when comparing sequential to parallel import
times. This is sufficient to verify parallel imports are working as
intended and should address the occasional false positive failure
when the time is slightly exceeded.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16638