From my experience, as far as Marketo is concerned once you send an email test, that "Email Subject Line Test" version is the new name of the asset itself, so what you're finding is likely correct.
Is that email asset the only one in the program? If so, you could use email "starts with" instead of "is" and capture any email assets within the program to cover your bases as well.
Additionally, to test this out, you could great a new Smart List, add the rules that you mentioned above "Not Opened Email > Starts With > Program.Asset.Name," and see if the number matches the unopened number from your email performance report.
Thank you, Chris.
In the past, I have been able to use both the subject line test and the email that was tested within the "Not Opened Email" filter.
It is not the only asset in the program and every other asset shows up in the drop down. I have tried to use "starts with" and it does not work.