add forward compatibility test

Author: r2k1

aks-node-controller/pkg/nodeconfigutils/backward_compat_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 // PopulateAllFields recursively populates all fields in a protobuf message with non-zero test values.
-// This is useful for testing to ensure all fields can be marshaled/unmarshaled correctly.
+// This is used for testing to ensure all fields can be marshaled/unmarshaled correctly.
 func PopulateAllFields(msg proto.Message) {
 	populateMessage(msg.ProtoReflect(), 0)
 }