LSM: SafeSetID: add selftest

This patch adds a selftest for the SafeSetID LSM. The test requires
mounting securityfs if it isn't mounted, creating test users in
/etc/passwd, and configuring policies for the SafeSetID LSM through
writes to securityfs.

Signed-off-by: Micah Morton <mortonm@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>