LU-3036 test: make atime update properly with 2.x servers

This patch includes the following fixes:
- set both MDS_ATTR_xTIME | MDS_ATTR_xTIME_SET when converting
  from OBD_MD_FLATIME in mdc_close_pack_20(). This will fix
  new 1.8 clients with old 2.x servers.
- improve sanityn.sh test_23
- remove sanity.sh test_203 since it does the same check to
  sanityn.sh test_23

Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: Ia5a2749dc548614d55d1b50a3ac57e34dfed56c4
Reviewed-on: http://review.whamcloud.com/6289
Tested-by: Hudson
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>