add unit tests for determining horizontal and vertical lines in lib.

TestCentre/LibraryTests.fs

@@ -93,6 +93,8 @@
 
         open TestingHelpers
         open Xunit
+        open DeathSocket
+        open System
 
         (* This test is a precaution (a test for the tests if you will...).
         It is here to make sure the property test has what it needs to run.
@@ -103,4 +105,14 @@
         let ``LoadingTestArea contains at least 100 test images`` () =
             let length = imagesInLoadingTestArea.Length
             let imagesAreThere = if length < 100 then false else true
-            Assert.True imagesAreThere
+            Assert.True imagesAreThere
+
+        [<Fact>]
+        let ``Divide By Zero Exception is thrown when 0 rows is used when determining horizontal lines`` () =
+            let result () = GridPainter.determineHorizontalLines 100 100 0
+            Assert.Throws<DivideByZeroException>(fun () -> result() |> ignore)
+
+        [<Fact>]
+        let ``Divide By Zero Exception is thrown when 0 columns is used when determining vertical lines`` () =
+            let result () = GridPainter.determineVerticalLines 100 100 0
+            Assert.Throws<DivideByZeroException>(fun () -> result() |> ignore)