powerpc/selftests: Add test for papr-sysparm

Consistently testing system parameter access is a bit difficult by
nature -- the set of parameters available depends on the model and
system configuration, and updating a parameter should be considered a
destructive operation reserved for the admin.

So we validate some of the error paths and retrieve the SPLPAR
characteristics string, but not much else.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-13-e9eafd0c8c6c@linux.ibm.com