Skip flaky system clock test

DesktopClock.Tests/SystemClockTests.cs

@@ -14,7 +14,7 @@ public class SystemClockTimerTests
         _timer = new SystemClockTimer();
     }
 
-    [Theory]
+    [Theory(Skip = "Relies on system performance")]
     [InlineData(3)]
     public async Task ShouldTickEverySecondAccurately(int seconds)
     {