[9.20] fix: test: The statschannel tests fails if one of libxml2 or json-c is configured

The `statschannel` system test failed if only one of `libxml2` or `json-c` is
available / configured as checks were being run against the non available
statistics page.

Closes #4919

Backport of MR !9454

Merge branch 'backport-4919-fix-statschannel-system-test-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9483