Add unit tests for FlowPanelDesigner (#11735)

* Add unit tests for FlowPanelDesigner

* update

* Remove panel returning

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/FlowPanelDesignerTests.cs

@@ -0,0 +1,59 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using Moq;
+using System.ComponentModel;
+using System.ComponentModel.Design;
+using System.Windows.Forms.Design.Behavior;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public sealed class FlowPanelDesignerTests
+{
+    private FlowPanelDesigner SetupDesignerWithPanel()
+    {
+        Mock<IDesignerHost> mockHost = new();
+        Mock<IContainer> mockContainer = new();
+        mockHost.Setup(host => host.Container).Returns(mockContainer.Object);
+
+        Panel panel = new();
+        mockContainer.Object.Add(panel);
+
+        FlowPanelDesigner designer = new();
+        designer.Initialize(panel);
+
+        return (designer);
+    }
+
+    [Fact]
+    public void ParticipatesWithSnapLines_ShouldAlwaysReturnFalse()
+    {
+        var designer = SetupDesignerWithPanel();
+
+        designer.ParticipatesWithSnapLines.Should().BeFalse();
+    }
+
+    [Fact]
+    public void SnapLines_ShouldNotContainPaddingSnapLines()
+    {
+        var designer = SetupDesignerWithPanel();
+
+        var snapLines = designer.SnapLines as IList<SnapLine>;
+
+        snapLines.Should().NotBeNull();
+        snapLines.Should().NotContain(line => line.Filter is object && line.Filter.Contains(SnapLine.Padding));
+    }
+
+    [Fact]
+    public void SnapLines_ShouldReturnNonPaddingSnapLines()
+    {
+        var designer = SetupDesignerWithPanel();
+
+        var snapLines = designer.SnapLines as IList<SnapLine>;
+
+        snapLines.Should().NotBeNull();
+        snapLines.Should().Contain(line => line.Filter == null || !line.Filter.Contains(SnapLine.Padding));
+    }
+}