Tests: namespace tests fail to build on older libc

The namespace tests fail to build on older libcs that don't define the
various namespace clone flags. To circumvent this problem, local
definitions from Linux's sched.h are added.

The kernel's support of the various namespaces is tested by the test
runner. Hence, it is not a problem to build these test application
against a kernel that isn't configured to support (some of) those
namespaces; the applications are not invoked for unsupported
namespaces.

Acked-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>