Skip Bitmap tests on non-Windows platforms

System.Drawing.Common requires GDI+ which is only available on Windows
since .NET 6. Add [ClassInitialize] with OperatingSystem.IsWindows()
check to all three Bitmap test classes so they report as Inconclusive
(skipped) on Linux/macOS CI runners instead of failing with
PlatformNotSupportedException.

Author: Nucs

test/NumSharp.UnitTest/NumSharp.Bitmap/BitmapExtensionsTests.cs

@@ -10,6 +10,13 @@ namespace NumSharp.UnitTest
     [TestClass]
     public class BitmapExtensionsTests : TestClass
     {
+        [ClassInitialize]
+        public static void RequireWindows(TestContext _)
+        {
+            if (!OperatingSystem.IsWindows())
+                Assert.Inconclusive("System.Drawing.Common requires Windows (GDI+).");
+        }
+
         // ================================================================
         // Bugs discovered during test coverage expansion:
         //

test/NumSharp.UnitTest/NumSharp.Bitmap/BitmapWithAlphaTests.cs

@@ -1,4 +1,5 @@
-using System.Drawing.Imaging;
+using System;
+using System.Drawing.Imaging;
 using System.Resources;
 using FluentAssertions;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
@@ -9,6 +10,13 @@ namespace NumSharp.UnitTest
     [TestClass]
     public class BitmapWithAlphaTests : TestClass
     {
+        [ClassInitialize]
+        public static void RequireWindows(TestContext _)
+        {
+            if (!OperatingSystem.IsWindows())
+                Assert.Inconclusive("System.Drawing.Common requires Windows (GDI+).");
+        }
+
         [TestMethod]
         public void ToNDArray_Case1()
         {

test/NumSharp.UnitTest/OpenBugs.Bitmap.cs

@@ -57,6 +57,13 @@ namespace NumSharp.UnitTest
     [TestClass]
     public class OpenBugsBitmap : TestClass
     {
+        [ClassInitialize]
+        public static void RequireWindows(TestContext _)
+        {
+            if (!OperatingSystem.IsWindows())
+                Assert.Inconclusive("System.Drawing.Common requires Windows (GDI+).");
+        }
+
         // ================================================================
         //
         //  BUG 1: ToNDArray(copy:true) on odd-width 24bpp images produces