Cannot save blank user_name on in Processor Details for Test Payments

c193819f was meant as a way to solve !6 (closed) , by allowing the admin to leave the API Login ID (username) blank if you don't want the checks to run.

However, I'm observing that it's not possible through the UI to leave this value blank, because blank values in the test processor section are filled in with equivalent values from the live processor section.

It's true that if civicrm_payment_process.user_name is blank, the checks will not run. However, the only way I can see to do this is by directly updating the column via sql.

Perhaps I'm missing something?