[9.20] new: test: Add isctest.check.ede() helper for pytest

Add a utility function to check for EDE codes present in the DNS
message. The primary benefit of this helper function is that it
handles the compatibility issues with different dnspython versions
and the actual test code doesn't have to deal with that any more.

Backport of MR !11182

Merge branch 'backport-nicki/isctest-check-ede-helper-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11233