ZTS: Avoid piping send directly to /dev/null

Unfortunately, #11445 means while we fail gracefully now, we still
fail, unless people want to implement a complex workaround just to
support /dev/null.

So let's just use the cheap workaround in a test for now.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12872