Does the acknowledgement arrive when someone withdraws?

Yes, it runs through your shop like a real one. Use a test order and mark the case as a test afterwards, so nobody triggers a refund.

Then place a cheap order with the test address first, and cancel it after the test.

Then your shop sends no acknowledgement, or it is stuck in your mail provider’s spam filter. You need to fix both: the acknowledgement is required.

It lands with my mail provider Resend and is only read automatically for this result. The test address is valid for two hours.

No. The test checks whether certain details are in the mail. Whether your whole process fits is in chapter 2.