Small enhancements to the layout testing engine

Source/QuestPDF.LayoutTests/TestEngine/LayoutTest.cs

@@ -146,7 +146,7 @@ internal class LayoutTest
         }
     }
 
-    public void ExpectLayoutException(string? reason = null)
+    public void ExpectLayoutException(string reason)
     {
         try
         {

Source/QuestPDF.LayoutTests/TestEngine/SolidBlock.cs

@@ -26,7 +26,7 @@ internal class SolidBlock : Element, IStateful
     internal override void Draw(Size availableSpace)
     {
         using var paint = new SkPaint();
-        paint.SetSolidColor(Colors.Grey.Medium);
+        paint.SetSolidColor(Placeholders.BackgroundColor());
         Canvas.DrawRectangle(Position.Zero, availableSpace, paint);
         
         IsRendered = true;