Expect.equalUnder fn

Apply a function to both `actual` and `expected` before comparing them:

```elm
expectEqualUnder : (a -> b) -> a -> a -> Expectation
expectEqualUnder f a2 a1 =
    f a1
        |> Expect.equal (f a2)
```

Example of usage:
```elm
str
  |> thereAndBack
  |> expectEqualUnder String.trim str
```