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.

``` unison
unique ability A where
  a : Nat

unique ability B where
  a : Char

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

``` ucm

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      ability A
      ability B
      test : '{B} Nat

```