Configuration form uses $config = $this->configFactory->getEditable('smtp.settings');. I suppose the motivation for not displaying possibly over-ridden values is that you might not want a user saving the form, and then exporting the over-ridden configuration. However, this means there is no way to visually see what values will actually be used when sending emails.
If variables have been over-ridden then the input is disabled. Could these over-ridden values (apart from password) not be displayed somewhere on the form?
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@patrickharris
I have exact same issue.
1. configure smtp module via settings.php
2. go to /admin/config/system/smtp
3. send test email
4. Receive "You must enter an SMTP server address" validation error.
Two potential solutions:
1. As you mention, move test form to a different tab/page.
2. Change form validation so it recognizes fields that are set via config and do not validate them.