tests: avoid false failure with --disable-selinux

tests: avoid false failure with --disable-selinux

* tests/ls/selinux-segfault.sh: Move recent addition to ...
* tests/ls/selinux.sh: ... this new test that uses require_selinux_
to skip appropriately when we've built without selinux support.
Also add a non root test that checks we output '.' along with the
mode for files, to indicate a security context is present.
* tests/local.mk (Reference the new test).