Write the first test

NightLight.Core.Tests/NightLightTests.fs

@@ -0,0 +1,20 @@
+module NightLight.Core.Tests.NightLightTests
+
+open FsCheck.Xunit
+
+let private assertIsOk (result: Result<unit, 'a>) : unit =
+    match result with
+    | Ok() -> ()
+    | Error error -> failwith $"Expected Ok, got Error {error}"
+
+[<Property(Arbitrary = [| typeof<Arbitraries> |])>]
+let ``Brightness should always be under 255`` (fakeHome: FakeHome) (interactions: Interaction list) =
+    interactions
+    |> Seq.iter (fun interaction -> fakeHome.Interact interaction |> assertIsOk)
+
+    fakeHome.LightStates
+    |> Seq.choose (fun (_, state) ->
+        match state with
+        | On brightness -> Some brightness
+        | Off -> None)
+    |> Seq.forall (fun brightness -> brightness < 255uy)