join-ns: Fix join-ns zdtm test

join-ns zdtm test will skip ns and uns flavors, and
some socket related test, too. Because join-ns test will
try to join an existing network namespace.

Signed-off-by: Deng Guangxing <dengguangxing@huawei.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>