Permit mocking stackable trait pattern in scala 3

[hughsimpson]

Pull Request Checklist

• I agree to licence my contributions under the MIT licence
• I have added copyright headers to new files
• I have added tests for any changed functionality

Fixes

Fixes mocking classes that implement the stackable trait pattern (i.e. abstract override def) in scala 3

[hughsimpson]

Maybe not the forum to say this, but other than #527 this was the only bug I've hit so far on migrating from scala 2 to scala 3, so absolutely incredible job on compatibility here.

[hughsimpson]

I appreciate there's a nuance here of 'do we really want to make it impossible to mock artifact methods?'. I think they're already essentially not unlockable? As in it would probably be pointless to try to use them in any mock. So I'm pretty sure that it's a legit forbid

[goshacodes]

In my opinion, we shouldn't allow mocking this.
The only things that should be mocked are traits or abstract classes not extending anything, because if you mock something else you are breaking Interface segregation principle and this leads you the wrong way.
But now we allowing to mock a lot of things, e.g. classes, so this can be legit

[hughsimpson]

@goshacodes sorry for taking so long to get to this, pushed the changes -- hadn't realised it was as simple as adding a new case to the MockableDefinitions constructor, nice catch. Added a couple more tests, too.

[goshacodes]

@barkhorn let's merge it