LU-18328 tests: fix old MDS version for sanity-sec/51

LU-18328 tests: fix old MDS version for sanity-sec/51

Fix the old MDS version for sanity-sec test 51 to skip the test
for symbolic capability names.

The patch also reverts the NODEMAP_TESTS change from commit
v2_16_0-RC1-11-g2a5e8e355498, which casued the following failure:
  EXCEPT_7=true: command not found

Test-Parameters: trivial testlist=sanity-sec env=ONLY=51 serverversion=2.15.5

Change-Id: Idf458ad5df7a7d6105b1f4da3e06280cf66919c1
Fixes: 0b0e9162e2 ("LU-13791 mdt: allow using symbolic capability names")
Fixes: 2a5e8e3554 ("LU-18247 nodemap: initialize unused fields on disk")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56624
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>