Revert "attempt to re-enable MultiplePoolInstances_TrimPeriodElapsed_AllAreTrimmed on Mac"

This reverts commit 7eaa5ee1a2.

tests/ImageSharp.Tests/Memory/Allocators/UniformUnmanagedMemoryPoolTests.Trim.cs

@@ -58,7 +58,11 @@ namespace SixLabors.ImageSharp.Tests.Memory.Allocators
             [Collection(nameof(NonParallelCollection))]
             public class NonParallel
             {
-                [Fact]
+                public static readonly bool IsNotMacOs = !TestEnvironment.IsOSX;
+
+                // TODO: Investigate failures on MacOS. All handles are released after GC.
+                // (It seems to happen more consistently on .NET 6.)
+                [ConditionalFact(nameof(IsNotMacOs))]
                 public void MultiplePoolInstances_TrimPeriodElapsed_AllAreTrimmed()
                 {
                     RemoteExecutor.Invoke(RunTest).Dispose();