tests: support non-MLS enabled SELinux systems

tests: support non-MLS enabled SELinux systems

When running "make check" on a Linux system running SELinux with a
non-MLS policy, tests/mkdir/restorecon.sh test fails with:

  chcon: invalid context: root:object_r:tmp_t:s0: Invalid argument

Indeed in such a configuration, contexts cannot have ":s0" suffix.

* init.cfg (get_selinux_type): Refactor this function to here
from various tests.  Update to work with a non-MLS policy.
(mls_enabled_): A new function to detect if MLS is enabled.
(skip_if_mcstransd_is_running_): Update to not skip when
MLS is not enabled.
* tests/mkdir/restorecon.sh: Use a valid non-MLS context when needed.
* tests/install/install-Z-selinux.sh: Likewise.
* tests/cp/cp-a-selinux.sh: Likewise.
* tests/misc/selinux.sh: Likewise.
* tests/misc/chcon.sh: Skip if non-MLS as --range used throughout.
Fixes http://bugs.gnu.org/22631