This is a test order, you will not be charged

Was having the exact same issue until I realized this isn't about being in beta or not. For me it was about migrating from IAB v2 to v3. Specifically the accounts you have added in your 'Play Console > Settings > Account details > Gmail accounts with testing access'. I had accounts in there long before alpha/beta was possible. After removing them + 3 hours or so to propagate, the testing message stopped showing.

Might be documented in the v3 in-app billing somewhere but I didn't see it.