chg: test: Properly process JSON and XML in tests

Processing JSON and XML using `grep` and `sed` is error-prone, overly
lax in some ways, overly strict in others, and neither accurate nor
expressive.  Use `jq` and `xmllint` with XPath expressions to make
things right in system tests.

See #3304

Merge branch '3304-properly-process-json-and-xml-in-tests' into 'main'

See merge request isc-projects/bind9!10942