ZTS: Provide an alternative to shuf for FreeBSD

There was a shuf package but the upstream for the port has recently
disappeared, so it is no longer available.

Create a function to hide the usage of shuf. Implement using seq|random
on FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9824

tests/zfs-tests/include/libtest.shlib

@@ -3934,3 +3934,18 @@ function faketty
         script --return --quiet -c "$*" /dev/null
     fi
 }
+
+#
+# Produce a random permutation of the integers in a given range (inclusive).
+#
+function range_shuffle # begin end
+{
+	typeset -i begin=$1
+	typeset -i end=$2
+
+	if is_freebsd; then
+		seq ${begin} ${end} | random -f -
+	else
+		shuf -i ${begin}-${end}
+	fi
+}