ZTS: tst.terminate_by_signal increase test threshold

The tst.terminate_by_signal test case may occasionally fail when
running in a less consistent virtual environment.  For all observed
failures the process was terminated correctly but it took longer than
expected resulting in too many snapshot being created.

To minimize the likelyhood of this occuring increase the threshold
from 50 to 90 snapshots.  The larger limit will still verifiy that
the channel program was correctly terminated early.

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9601

tests/zfs-tests/tests/functional/channel_program/synctask_core/tst.terminate_by_signal.ksh

@@ -91,7 +91,7 @@ log_note "$snap_count snapshots created by ZCP"
 
 if [ "$snap_count" -eq 0 ]; then
 	log_fail "Channel program failed to run."
-elif [ "$snap_count" -gt 50 ]; then
+elif [ "$snap_count" -gt 90 ]; then
 	log_fail "Too many snapshots after a cancel ($snap_count)."
 else
 	log_pass "Canceling a long-running channel program works."