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.

Merge branch 'stepan/avoid-using-hand-rolled-responses-in-asyncserver' into 'main'

See merge request isc-projects/bind9!11420