tests: fix false failure with disabled SELinux support
This could trigger on SELinux systems where we build --qithout-selinux
or where the SELinux development libraries are not installed.
* init.cfg (require_selinux_enforcing_): Call require_selinux_()
to determine if the current build supports SELinux. This avoids
a false failure in tests/mkdir/selinux.sh where only mkdir would
determine that SELinux was disabled and thus ignore invalid contexts.
(require_selinux_): Refactor a little to distinguish whether it's
the build or the (file) system that doesn't support SELinux.