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.

Fixes: f6687bafcb ("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