Illumos #1950: ztest backwards compatibility testing option.

Illumos #1950: ztest backwards compatibility testing option.

illumos/illumos-gate@420dfc9585ff67e83ee7800a7ad2ebe1a9145983
Illumos changeset: 13571:a5771a96228c

1950 ztest backwards compatibility testing option

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Eric Schrock <eric.schrock@delphix.com>

Ported-by: Etienne Dechamps <etienne.dechamps@ovh.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>