Disable a SignalAndWait test so that relevant change in CoreCLR can be brought into CoreFX (#27131)

Author: kouvel

src/System.Runtime/tests/System/Threading/WaitHandleTests.cs

@@ -320,17 +320,6 @@ public static void SignalAndWait_InvalidArgs()
             Assert.False(toSignal.WaitOne(0));
         }
 
-        [Fact]
-        [PlatformSpecific(TestPlatforms.AnyUnix)]  // Unix doesn't support SignalAndWait
-        public static void SignalAndWait_PlatformNotSupported()
-        {
-            var toSignal = new ManualResetEvent(false);
-            var toWaitOn = new ManualResetEvent(true);
-            Assert.Throws<PlatformNotSupportedException>(() => WaitHandle.SignalAndWait(toSignal, toWaitOn));
-            Assert.Throws<PlatformNotSupportedException>(() => WaitHandle.SignalAndWait(toSignal, toWaitOn, 0, false));
-            Assert.Throws<PlatformNotSupportedException>(() => WaitHandle.SignalAndWait(toSignal, toWaitOn, TimeSpan.Zero, false));
-        }
-
         [Fact]
         public static void Close()
         {