tests: avoid false failure when restorecon is ineffective

tests: avoid false failure when restorecon is ineffective

* tests/cp/cp-a-selinux.sh: Ensure we skip the portion of the test
depending on restorecon to be effective.  I.E. also skip when restorecon
warns, as it doesn't exit with error status when matchpathcon fails to
find a match for a file.  This is the case in /tmp on Fedora for
example, in which case the new destination that cp creates will have the
default security context of the process, rather than the explicit
context we set on the source file.
Details at: http://bugzilla.redhat.com/1247641