[tests] add tests for NAT64 prefix advertisement (#7361)

This commit adds tests for NAT64 prefix
advertisement. BORDER_ROUTING_NAT64 is set to 1 to enable the feature
in tests.

It also adjusted some util functions for prefixes and routes in
netdata.

Testing outbound connectivity to IPv4 hosts is not covered yet. It
will be added after we update OTBR.

script/test

@@ -46,6 +46,7 @@ readonly THREAD_VERSION="${THREAD_VERSION:-1.2}"
 readonly INTER_OP="${INTER_OP:-0}"
 readonly VERBOSE="${VERBOSE:-0}"
 readonly BORDER_ROUTING="${BORDER_ROUTING:-1}"
+readonly BORDER_ROUTING_NAT64="${BORDER_ROUTING_NAT64:-1}"
 readonly INTER_OP_BBR="${INTER_OP_BBR:-1}"
 
 readonly OT_COREDUMP_DIR="${PWD}/ot-core-dump"
@@ -295,6 +296,12 @@ do_build_otbr_docker()
         otbr_options+=("-DOTBR_TREL=OFF")
     fi
 
+    if [[ ${BORDER_ROUTING_NAT64} == 1 ]]; then
+        otbr_options+=("-DOT_BORDER_ROUTING_NAT64=ON")
+    else
+        otbr_options+=("-DOT_BORDER_ROUTING_NAT64=OFF")
+    fi
+
     local otbr_docker_image=${OTBR_DOCKER_IMAGE:-otbr-ot12-backbone-ci}
 
     otbrdir=$(mktemp -d -t otbr_XXXXXX)