Fix failing tests.

`ScrollContentPresenter` now coerces offset so it can only be set after a viewport has been calculated.

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests.cs

@@ -117,13 +117,17 @@ namespace Avalonia.Controls.UnitTests.Presenters
                     Width = 150,
                     Height = 150,
                 },
-                Offset = new Vector(25, 25),
             };
 
             target.UpdateChild();
             target.Measure(new Size(100, 100));
             target.Arrange(new Rect(0, 0, 100, 100));
 
+            target.Offset = new Vector(25, 25);
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
             Assert.Equal(new Rect(-25, -25, 150, 150), content.Bounds);
         }
 
@@ -345,4 +349,4 @@ namespace Avalonia.Controls.UnitTests.Presenters
             }
         }
     }
-}
+}

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests_ILogicalScrollable.cs

@@ -59,13 +59,17 @@ namespace Avalonia.Controls.UnitTests
                 CanHorizontallyScroll = true,
                 CanVerticallyScroll = true,
                 Content = scrollable,
-                Offset = new Vector(25, 25),
             };
 
             target.UpdateChild();
             target.Measure(new Size(100, 100));
             target.Arrange(new Rect(0, 0, 100, 100));
 
+            target.Offset = new Vector(25, 25);
+            
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
             Assert.Equal(new Rect(-25, -25, 150, 150), scrollable.Bounds);
         }