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.
In order to run the shell system tests, the pytest runner has to pick
them up somehow. Adding an extra python file with a single function
for the shell tests for each system test proved to be the most
compatible way of running the shell tests across older pytest/xdist
versions.
Modify the legacy run.sh script to ignore these pytest-runner specific
glue files when executing tests written in pytest.
