selftests: add PM netlink functional tests

This introduces basic self-tests for the PM netlink,
checking the basic APIs and possible exceptional
values.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>