Tests for a TDNR case that wasn't working. The code wasn't 'relaxing'
inferred types that didn't contain arrows, so effects that just yield
a value weren't getting disambiguated.

```ucm:hide
scratch/main> builtins.merge
```

```unison
unique ability A where
  a : Nat

unique ability B where
  a : Char

test : () -> Nat
test _ =
  x = a
  toNat x
```