Maintaining compatibility with pre-2.0.0 dnspython became cumbersome
leading to failure in nightly CI jobs which are the only ones that run
with dnspython this old.
Abort all AsyncServer instances when running with old dnspython. Add an
importor skip for all system tests using isctest.asyncserver.
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.
