usesFloat incorrectly identifies union type as a float

The logic `elm-test` uses to identify floats incorrectly identifies a union type with a potential value of `Infinity` as a float. This is because `String.toFloat "Infinity" == Just Infinity`.

This error *only* happens when the name of the union potential value is `Infinity`.

# SSCE
```elm
module Example exposing (..)

import Expect exposing (Expectation)
import Fuzz exposing (Fuzzer, int, list, string)
import Test exposing (..)

type UnionType =
Infinity

suite : Test
suite =
test "should not give a 'use float' error" <|
\() ->
Infinity
|> Expect.equal Infinity

```

Running `elm-test` with the latest (beta) version gives:
```
Running 1 test. To reproduce these results, run: elm-test --fuzz 100 --seed 241676437961883

↓ Example
✗ should not give a 'use float' error

Do not use Expect.equal with floats. Use Float.within instead.
```

I do not know what the solution is for this, I just renamed my union types as a work around.