Add workaround for tests on SELinux systems.

This change contains some simple workarounds for the unit tests, in the case where you're on an SELinux system where temporary test files get the security.selinux attribute. My fix is a bit hacked together, and I haven't tested it thoroughly (Python 2.7 and 3.3, but only on the one system). But I think the idea is basically sound; if you define HAVE_SELINUX=TRUE, the assertions will either expect or ignore security.selinux.