Use isctest.asyncserver in the "forward" test

Use isctest.asyncserver in the "forward" test

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.