Get rid of arbitraries that are only used once

NightLight.Core.Tests/TimeChangedGenerators.fs

@@ -0,0 +1,20 @@
+module NightLight.Core.Tests.TimeChangedGenerators
+
+open System
+open FsCheck.FSharp
+
+let private isDay (time: DateTime) =
+    time.TimeOfDay >= TimeSpan.FromHours 5.5
+    && time.TimeOfDay < TimeSpan.FromHours 20.5
+
+let genTimeChangedToDay =
+    ArbMap.defaults
+    |> ArbMap.generate<DateTime>
+    |> Gen.filter isDay
+    |> Gen.map Interaction.TimeChanged
+
+let genTimeChangedToNight =
+    ArbMap.defaults
+    |> ArbMap.generate<DateTime>
+    |> Gen.filter (not << isDay)
+    |> Gen.map Interaction.TimeChanged