test: don't use nsenter in tests

Ubuntu 14.04 (Travis) doesn't have it.
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Looks-good-to: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/empty-netns-prep.sh

-#!/bin/sh
+#!/bin/bash
+
+set -ex
 
 if [ "$CRTOOLS_SCRIPT_ACTION" == "setup-namespaces" ]; then
 	echo "Will up lo at $CRTOOLS_INIT_PID netns"
-	nsenter -t "$CRTOOLS_INIT_PID" --net ip link set up dev lo || exit 1
+	mkdir -p /var/run/netns
+	mount -t tmpfs xxx /var/run/netns
+	touch /var/run/netns/emptyns
+	mount --bind /proc/$CRTOOLS_INIT_PID/ns/net /var/run/netns/emptyns
+	ip netns exec emptyns ip link set up dev lo || exit 1
+	ip netns exec emptyns ip a
+	umount -l /var/run/netns
 fi
 
 exit 0