LU-17317 sec: fix sanity-sec test_28

LU-17317 sec: fix sanity-sec test_28

Improve sanity-sec test_28 to verify that srpc_contexts is valid
YAML output.
Also remove the ctx information from the output, as printing out a
kernel pointer is not ideal.

Lustre-change: https://review.whamcloud.com/54280
Lustre-commit: TBD (from 7ff01038c81a813cd122b28d21964316e7afdf28)

Fixes: 45f2b1ca36 ("LU-17317 sec: add srpc_serverctx proc file")
Test-Parameters: trivial
Test-Parameters: kerberos=true testlist=sanity-krb5
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ie48dc61adfd5017a2313981f27407c9d3b69dd71
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54281
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>