FWIW, I don't mind having separate tests like that for now, then I think they could also be part of the `test_wmv_decoder` tests. That could be a refactor for later though, once we have a better test coverage of interfaces, and for more decoders (the WMA decoder needs it too).
Ultimately it may be interesting to check if the dual decoders supports some interoperability between both their interfaces, at least to see if we can/should implement them separately or rather on top of each other.
Also, do you intend to implement more of this? FWIW Proton has the DMO interfaces implemented for WMV and WMA, some games required them, though it's missing tests entirely.
I wanted to write tests but had some other things pending and got side tracked a bit. If you're interested I'm happy to leave it to you :).
Looks good otherwise.