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 openzfs#16638

tests/zfs-tests/tests/functional/cli_root/zpool_import/zpool_import_parallel_pos.ksh

@@ -113,7 +113,7 @@ wait
 parallel_time=$SECONDS
 log_note "asyncronously imported 4 pools in $parallel_time seconds"
 
-log_must test $parallel_time -lt $(($sequential_time / 3))
+log_must test $parallel_time -lt $(($sequential_time / 2))
 
 #
 # export pools with import delay injectors
@@ -132,6 +132,6 @@ log_must zpool import -a -d $DEVICE_DIR -f
 parallel_time=$SECONDS
 log_note "asyncronously imported 4 pools in $parallel_time seconds"
 
-log_must test $parallel_time -lt $(($sequential_time / 3))
+log_must test $parallel_time -lt $(($sequential_time / 2))
 
 log_pass "Pool imports occur in parallel"