Unit testing challenge

challenge.md

I'm looking for the cleanest way to make the test below pass.

The rules are:

1. It must be a unit test (so don't touch the file system).
2. The public signature for CodeUnderTest must not be changed. can be extended not reduced
3. You're allowed to use DI/your favorite mocking framework.
4. You can only add code where you see /**/.

        [Test]
        public void Test()
        {
            /**/
            var text = CodeUnderTest.GetUpperText("foo.txt" /**/);

            Assert.That(text, Is.EqualTo("BAR"));
            /**/
        }

        public class CodeUnderTest
        {
            /**/
            public static string GetUpperText(string path /**/)
            {
                var text = File.ReadAllText(path);
                return text.ToUpperInvariant();
            }
            /**/
        }

Please comment below or tweet me @jcansdale.

How would that look in F#? That question triggered a blog post, but the summary is this:

// string -> string
let getUpperText path =
    path
    |> Some
    |> Option.filter File.Exists
    |> Option.map (File.ReadAllText >> getUpper)
    |> Option.defaultIfNone "DEFAULT"

Apart from getUpper there's nothing else to test 😉