LU-9054 tests: disable test_312 due to zdb issue

LU-9054 tests: disable test_312 due to zdb issue

zdb used to work for datasets of exported pool by '-e -p' options,
this has been changed in zfs-0.7.0.

This patch temporarily disables test_312 until zfs upstream ticket
https://github.com/zfsonlinux/zfs/issues/6464 is solved.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ib0c9eeed4964ea4a0abfed70760cb8fbaeb44496
Reviewed-on: https://review.whamcloud.com/28343
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>