How to test Google Play In-app subscription immediately?

I have an annoying trouble in testing Free trial subscription. My gmail has already been added to license testers' list in my play console. But I can't figure out exactly when it becomes available.

I know that there are certain renewal periods for license test, and it can be renewed 6 times at maximum. But it just used to become available unexpectedly. What I am looking for is a way to test free-trial subscription immediately whenever I need test.

Any help or hint would be appreciated.