Fixed test regression on Windows and FreeBSD

Author: sakno

src/DotNext.Tests/Buffers/UnmanagedMemoryPoolTests.cs

@@ -276,13 +276,15 @@ public static void AllocatePageAlignedMemoryInvalidSize()
         Equal(Memory<byte>.Empty, owner.Memory);
     }
 
-    [Fact]
+    [PlatformSpecificFact("linux", "windows")]
     public static void DiscardPages()
     {
         using var systemPages = UnmanagedMemory.AllocateSystemPages(1);
         systemPages.Memory.Span[0] = 42;
 
+        // On FreeBSD and MacOS, the method doesn't clear the memory
         UnmanagedMemory.Discard(systemPages.Memory.Span);
+        
         Equal(0, systemPages.Memory.Span[0]);
     }
 

src/DotNext.Unsafe/Buffers/UnmanagedMemory.cs

@@ -302,7 +302,7 @@ internal static void Discard(nint address, nint length)
             return;
 
         int errorCode;
-        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS() || OperatingSystem.IsFreeBSD())
         {
             const int MADV_DONTNEED = 4;