[test] add tests for DNS forwarding (#8847)

This commit adds a new file
`tests/scripts/thread-cert/border_router/nat64/test_upstream_dns.py`
to test scripts for testing the DNS forwarding functions.

The test adds a record `test.domain` to the bind9 interface since when
adding to DNSSD tests, the OpenThread's DNSSD server will be conflict
with the Docker's DNS forwarding functions.

script/test

@@ -321,8 +321,8 @@ do_build_otbr_docker()
         "-DOT_SRP_CLIENT=ON"
         "-DOT_FULL_LOGS=ON"
         "-DOT_UPTIME=ON"
+        "-DOTBR_DNS_UPSTREAM_QUERY=ON"
         "-DOTBR_DUA_ROUTING=ON"
-        "-DCMAKE_CXX_FLAGS='-DOPENTHREAD_CONFIG_DNSSD_SERVER_BIND_UNSPECIFIED_NETIF=1'"
     )
     local args=(
         "BORDER_ROUTING=${BORDER_ROUTING}"
@@ -338,6 +338,11 @@ do_build_otbr_docker()
         "MDNS=${OTBR_MDNS:-mDNSResponder}"
     )
 
+    if [[ ${NAT64} != 1 ]]; then
+        # We are testing upstream DNS forwarding in the NAT64 tests, and OPENTHREAD_CONFIG_DNSSD_SERVER_BIND_UNSPECIFIED_NETIF will block OpenThread's DNSSD server since we already have bind9 running.
+        otbr_options+=("-DCMAKE_CXX_FLAGS='-DOPENTHREAD_CONFIG_DNSSD_SERVER_BIND_UNSPECIFIED_NETIF=1'")
+    fi
+
     if [[ ${TREL} == 1 ]]; then
         otbr_options+=("-DOTBR_TREL=ON")
     else