add scaling pen thickness tests.

TestCentre/LibraryTests.fs

@@ -421,6 +421,24 @@
             let imagesAreThere = if length < 100 then false else true
             Assert.True imagesAreThere
 
+        [<Fact>]
+        let ``Pen thickness scales down to match preview image dimensions`` () =
+            let result () =
+                GridPainter.scaleLineThickness (100, 100) (200, 200) 10.0
+            Assert.Equal (5.0, (result ()))
+
+        [<Fact>]
+        let ``Pen thickness scales up to match preview image dimensions`` () =
+            let result () =
+                GridPainter.scaleLineThickness (200, 200) (100, 100) 10.0
+            Assert.Equal (20.0, (result ()))
+
+        [<Fact>]
+        let ``Pen thickness remains the same when preview image matches actual dimensions`` () =
+            let result () =
+                GridPainter.scaleLineThickness (100, 100) (100, 100) 10.0
+            Assert.Equal (10.0, (result ()))
+
         (* This test is a pre-test test. If the property tests fails, here is a
         good place to start. The easiest way to get this test to pass is to
         create a folder called "SavingTestArea" in this projects folder