`defaults` is the default values e.g. `1` in `foo=1`. It's just used to
find the offset of the kwargs, but the code reused the name which made
me scratch my head for a minute.

The test passes invalid values to the instance (it does not takes
functions, it takes HookImpls). The test fails, but it accidentally used
`return` instead of `assert` so it wasn't visible.

Since what is being tested is evidently unimportant and legacy, just
remove it.

It doesn't take `**kwargs` but an argument `kwargs`.