Using Keyman Developer 10, you have the option to select the underlying system keyboard when testing/debugging a new keyboard. However, this seems to have no effect - the keyboard doesn’t change, and when you repeat the process it seems the first keyboard in the list is selected. I am not sure if this is the place to record this.
Thanks for reporting this functionality problem. I’ve noted this as an issue in our issue tracker along with a workaround: https://github.com/keymanapp/keyman/issues/1225
Thanks for the updated.