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/commands.cfg

@@ -82,7 +82,6 @@ export SYSTEM_FILES_COMMON='arp
     seq
     setfacl
     sh
-    shuf
     sleep
     sort
     ssh
@@ -122,6 +121,7 @@ export SYSTEM_FILES_FREEBSD='chflags
     mkfifo
     newfs
     pw
+    random
     sha256
     swapctl
     sysctl
@@ -157,6 +157,7 @@ export SYSTEM_FILES_LINUX='attr
     setenforce
     setfattr
     sha256sum
+    shuf
     udevadm
     useradd
     userdel