example implementation for fail_if_unknown

[sstrong99]

Checklist

• Pull request details were added to HISTORY.rst

This is incomplete. Just posting to demonstrate a possible implementation of the feature requested here: #182

[rhpvorderman]

As mentioned in #182, this is too specific to your needs. The custom test interface was created precisely for these reason. To allow more flexibility for specific user needs.
Given that you already have the YAML, it is trivial to parse it yourself, get each workflow name and all the defined files. Then you can use the custom test interface to implement a test where these files are tested.