Disable IOCP config var test (#106888)

The test is still failing occasionally. A simplified version is failing more frequently locally, needs further investigation to determine the cause.

Tracking: #106371

src/libraries/System.Threading.ThreadPool/tests/ThreadPoolTests.cs

@@ -1342,6 +1342,7 @@ from x in Enumerable.Range(0, 9)
         [ConditionalTheory(nameof(IsThreadingAndRemoteExecutorSupported), nameof(UsePortableThreadPool))]
         [MemberData(nameof(IOCompletionPortCountConfigVarTest_Args))]
         [PlatformSpecific(TestPlatforms.Windows)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/106371")]
         public static void IOCompletionPortCountConfigVarTest(int ioCompletionPortCount)
         {
             // Avoid contaminating the main process' environment