Add test covering decoupled clock not drifting

Author: peppy

osu.Framework.Tests/Clocks/DecouplingFramedClockTest.cs

@@ -432,6 +432,23 @@ public void TestBackwardPlaybackOverZeroBoundary()
             }
         }
 
+        [Test]
+        public void TestNoDecoupledDrift()
+        {
+            var stopwatch = new StopwatchClock();
+
+            decouplingClock.Start();
+            stopwatch.Start();
+
+            decouplingClock.Seek(-100);
+            stopwatch.Seek(-100);
+
+            while (decouplingClock.CurrentTime <= 0)
+                decouplingClock.ProcessFrame();
+
+            Assert.That(decouplingClock.CurrentTime, Is.EqualTo(stopwatch.CurrentTime).Within(1));
+        }
+
         [Test]
         public void TestForwardPlaybackOverZeroBoundary()
         {