Hi Ben,
The way Authorize.NET has traditionally worked, it takes 2 settings to make a transaction" "real".
1: You Authorize.NET account is NOT in Test mode. This is set on your Authorize.NET account.
2: The "TestMode" flag is not passed to Authorize.NET during the call. This is what the flag in Kentico controls.
If either of these are set to "Test" mode, the transaction will not be "real". This allows you to use a "live" Auhtorize.NET account but still do a test transaction.
From personal experience, I have often worried a little whenever I I did a "test" transaction for the exact reasons you stated. I can tell you I have done many test transactions using live accounts and used the "test mode" flag to ensure it wasn't actually processed.
If you are really concerned, you could make a product $.01 and do a test purchase. In the off chance you have something not configured correctly, it will not be a major transaction Regardless of the amount, you can still void the transaction in your Authorize.NET account.