kernel/selinux-testsuite: move keys test exclude under RHEL-8
    
    The relevant config option is now enabled on RHEL-9, so we can skip it
    on RHEL-8 only to get coverage on RHEL-9 and above.
    
    Tested on latest RHEL-9 VM image.
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>