Test command line tools without stdio fds

Test command line tools without stdio fds

Testing all combinations seems unnecessary but is cheap.
I was too lazy to run this against all tools we have. nsupdate was
chosen because it is one of few tools which actually use stdin and the
original issue was reproducible even without any network communication,
which was not the case for simple dig invocation.

Sorry for new shell test but doing this in Python seemed very
complicated and fragile.