The Test button appears as soon as a URL is specified.
The test executes on the current selected Action. If the selected Action has a chained action then this will also be executed after the parent is done.
When testing a chained action that used Response Variables, then always select the parent and press Test. Since then the response can be parsed on the chained action.
Pressing Test directly on the Chained action will run the request but it does not know anything about the previous response.