tests: rsend.kshlib: cmp_ds_prop: allow skipping source

tests: rsend.kshlib: cmp_ds_prop: allow skipping source

This fixes rsend_012_pos:
20:28:50.50 SUCCESS: eval zfs receive -d -F testpool2 < /mnt/testroot/backdir-rsend/pool-final-R
20:28:50.53 4,6c4,6
20:28:50.53 < acltype   off     local
20:28:50.53 < dnodesize 4k      local
20:28:50.53 < atime     off     local
20:28:50.53 ---
20:28:50.53 > acltype   off     received
20:28:50.53 > dnodesize 4k      received
20:28:50.53 > atime     off     received
20:28:50.53 8,13c8,13
20:28:50.53 < checksum  sha256  local
20:28:50.53 < compression       off     local
20:28:50.53 < copies    2       local
20:28:50.53 < devices   on      local
20:28:50.53 < exec      on      local
20:28:50.53 < quota     1G      local
20:28:50.53 ---
20:28:50.53 > checksum  sha256  received
20:28:50.53 > compression       off     received
20:28:50.53 > copies    2       received
20:28:50.53 > devices   on      received
20:28:50.53 > exec      on      received
20:28:50.53 > quota     1G      received
20:28:50.53 15c15
20:28:50.53 < recordsize        128K    local
20:28:50.53 ---
20:28:50.53 > recordsize        128K    received
20:28:50.53 17,18c17,18
20:28:50.53 < setuid    off     local
20:28:50.53 < snapdir   visible local
20:28:50.53 ---
20:28:50.53 > setuid    off     received
20:28:50.53 > snapdir   visible received
20:28:50.53 ERROR: cmp_ds_prop testpool testpool2 exited 1

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13250
Closes #13259