tests: Flesh out securityselinuxhelper
    
    Add fake implementations of:
    
    - is_selinux_enabled
    - security_disable
    - selinux_virtual_domain_context_path
    - selinux_virtual_image_context_path
    - selinux_lxc_contexts_path
    - selabel_open
    - selabel_close
    - selabel_lookup_raw
    
    The selabel_* functions back onto the real implementations if SELinux is
    enabled on the test system, otherwise we just implement a fake selabel
    handle which errors out on all labelling lookups.
    
    With these changes in place, securityselinuxtest and
    securityselinuxlabeltest don't need to skip all tests if SELinux isn't
    available; they can exercise much of the security manager code.
    
    Signed-off-by: Michael Chapman <mike@very.puzzling.org>