Replace the custom DNS servers used in the "forward" system test with
new code based on the isctest.asyncserver module.
For ans6, instead of configuring the responses to send at runtime, set
them up when the server is started. Make sure the server supports
toggling response sending at runtime to enable simulating forwarder
timeouts as required by one of the checks.
For ans11, put most of the responses to be provided by that server into
a zone file, only retaining code modifying zone-based answers in the
form of a response handler, to improve code readability. Use explicit
domain names instead of variables as that server only handles a single
domain and fixed strings improve readability in this case. Make sure
the server supports toggling response sending at runtime to enable
simulating forwarder timeouts as required by one of the checks.
Migrate sendcmd() and its uses to the new way of sending control queries
to custom servers used in system tests.
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.
