Port TabTests and merge Dim.FillTests, remove empty ThemeTests

Co-authored-by: tig <585482+tig@users.noreply.github.com>

Tests/UnitTestsParallelizable/View/Layout/Dim.FillTests.cs

@@ -161,4 +161,19 @@ public class DimFillTests (ITestOutputHelper output)
         Assert.True (view.IsInitialized);
         Assert.Equal (expectedViewBounds, view.Viewport);
     }
+
+    [Fact]
+    public void DimFill_SizedCorrectly ()
+    {
+        var view = new View { Width = Dim.Fill (), Height = Dim.Fill (), BorderStyle = LineStyle.Single };
+        var top = new Toplevel ();
+        top.Add (view);
+
+        top.Layout ();
+
+        view.SetRelativeLayout (new (32, 5));
+        Assert.Equal (32, view.Frame.Width);
+        Assert.Equal (5, view.Frame.Height);
+        top.Dispose ();
+    }
 }