zdtm: Write netns test in

It was written but wasn't included into standard run because
ip tool with needed functionality wasn't available "upstream".
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -124,6 +124,7 @@ static/sem
 transition/ipc
 ns/static/tun
 static/netns-nf
+static/netns
 "
 
 TEST_CR_KERNEL="