Have a look at the workflow and look at what "event" is fired when the issue goes from <something> to Testing.
Once you know what the event is, you can add the "tester" role to the notification scheme (you could hard-code a name or group in the notification scheme, but it's better to use a role so that your users can select who are testers in the project)
If you don't have a "simplified" workflow, then you have a better option - you can specifically add an event called "moved to testing" and change the workflow so that that event is fired instead. Then, again, notify the test role. This has the advantage that only "testing" emails go out - you can separate out things like "issue updated" and "issue resolved" and not bother the testers with them, and equally not bother other users with "testing" emails.
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