lxc-attach: update and improve tests
lxc-attach: update and improve tests

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@mailbox.org>

aeris/lxc,caglar10ur/lxc,federicobriata/lxc,federicobriata/lxc,GreatFruitOmsk/lxc,hfuCN/lxc,aeris/lxc,ss1h2a3tw/lxc,LynxChaus/lxc,terceiro/lxc,hfuCN/lxc,ss1h2a3tw/lxc,akshaykarle/lxc,thmo/lxc,federicobriata/lxc,hfuCN/lxc,aeris/lxc,ss1h2a3tw/lxc,LynxChaus/lxc,ysbnim/lxc,thmo/lxc,tych0/lxc,tych0/lxc,ysbnim/lxc,QingweiPeterLan/cs188-lxc,LynxChaus/lxc,akshaykarle/lxc,akshaykarle/lxc,caglar10ur/lxc,GreatFruitOmsk/lxc,hallyn/lxc,tych0/lxc,caglar10ur/lxc,cjwatson/lxc,hallyn/lxc,ysbnim/lxc,cjwatson/lxc,terceiro/lxc,GreatFruitOmsk/lxc,QingweiPeterLan/cs188-lxc,hallyn/lxc,terceiro/lxc,thmo/lxc,cjwatson/lxc,QingweiPeterLan/cs188-lxc