Use [ConditionalFact] instead of [PlatformSpecific]

tests/ImageSharp.Tests/Memory/Allocators/UniformUnmanagedPoolMemoryAllocatorTests.cs

@@ -163,8 +163,9 @@ namespace SixLabors.ImageSharp.Tests.Memory.Allocators
             }
         }
 
-        [Fact]
+        public static bool IsNotOsx = !TestEnvironment.IsOSX;
-        [PlatformSpecific(~TestPlatforms.OSX)] // TODO: Investigate OSX failure
+
+        [ConditionalFact(nameof(IsNotOsx))] // TODO: Investigate OSX failure
         public void MemoryAllocator_Create_LimitPoolSize()
         {
             RemoteExecutor.Invoke(RunTest).Dispose();