Add unit tests for NumericUpDownAcceleration (dotnet#12705)

Related dotnet#10453

Proposed changes
Add unit tests for NumericUpDownAcceleration.cs to test its properties & methods
Enable nullability in NumericUpDownAccelerationTests.cs

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/NumericUpDownAccelerationTests.cs

@@ -0,0 +1,37 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+namespace System.Windows.Forms;
+
+public class NumericUpDownAccelerationTests
+{
+    [WinFormsFact]
+    public void NumericUpDownAcceleration_SecondsProperty_WorksAsExpected()
+    {
+        NumericUpDownAcceleration acceleration = new(5, 1.0m);
+
+        acceleration.Seconds.Should().Be(5);
+
+        acceleration.Seconds = 10;
+        acceleration.Seconds.Should().Be(10);
+
+        Action act = () => acceleration.Seconds = -1;
+        act.Should().Throw<ArgumentOutOfRangeException>();
+    }
+
+    [WinFormsFact]
+    public void NumericUpDownAcceleration_IncrementProperty_WorksAsExpected()
+    {
+        NumericUpDownAcceleration acceleration = new(5, 1.0m);
+
+        acceleration.Increment.Should().Be(1.0m);
+
+        acceleration.Increment = 2.0m;
+        acceleration.Increment.Should().Be(2.0m);
+
+        Action act = () => acceleration.Increment = -1.0m;
+        act.Should().Throw<ArgumentOutOfRangeException>();
+    }
+}