Please note that there seem to be differences between version 2.3 and 2.4.
I haven’t tested it with 2.4 yet, but in 2.3 you still have to enter a contact so that the notification is sent to Teams:
And keep in mind that “Test Notification” does not send a notification to teams, but only checks whether rules apply.
To actually send test notifications via this notification method, you must continue to use Fake check results in Checkmk 2.3.0.