Hello!
As a totally newbie I was testing 'mount' command but it doesn't worked for me.
then finally I used this commend above 'mount /test / ' - and sever doesn't respond now 
Is it possible to undo somehow this commanand?
Login to hardware console or plug a monitor to it, then try to do umount /test
If it'll not work, just reboot.
And next time, read manual of command before executing, just to be sure what is it doing.
since the binaries you need to unmount /test from / are overmounted, you need to reboot the system
I think even if you can get the umount binary to the /test Filesystem, it would not be possible to unmount it anyway
I've restarted. It works.
Thank you for your advices!
This situation costed me a lot of nerves.
Too funny. 
Typical mistake. It was funny but that's the way to learn new things!! You have learned "mount" the hard way. Just make sure you do not do these R&Ds on a production machine.