Add a regression test for `INothing` implicit resolution

[armanbilge]

We had a good one in https://github.com/typelevel/feral/pull/52#issuecomment-974246880, but then I went and changed the lambda encoding in #60. So would be good to make a new one. I.e. something that works for INothing but doesn't work for vanilla Nothing, but maybe that's too ambitious :P in which case an ordinary is just fine :)