[9.20] chg: test: Avoid sending manually created responses in asyncserver

[9.20] chg: test: Avoid sending manually created responses in asyncserver

If at all possible, all the responses should be created by
AsyncDnsServer's internal methods. To ensure this, mark them with a
magic attribute and check it on send and crash the server if a manually
created response is detected.

Backport of MR !11420

Merge branch 'backport-stepan/avoid-using-hand-rolled-responses-in-asyncserver-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11445