Add const_fn in generics test
Adds a test that constant functions in generic parameters work properly. I was surprised this works, but I also to turbofish the constant in main, otherwise it didn't infer properly:
```
let v: ConstU32<3> = ...
```
Did not work as I expected, which I can highlight in the test if that's the intended behaviour.
r? @lcnr
c3f842baee