stress_namespaces: make sure to fork after unshare() for pidns

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
moving stress_namespaces to a different directory

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
stress_namespaces: return error code immediately if test fails

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
stress_namespaces: add test system files

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>