Mark test failing on macOS runners as flaky

Another that fails is osu.Framework.Tests.Visual.Containers.TestSceneCachedBufferedContainer - TestConstructor, but it's not simple to add flaky there.

osu.Framework.Tests/Clocks/DecouplingFramedClockTest.cs

@@ -436,6 +436,7 @@ namespace osu.Framework.Tests.Clocks
         [TestCase(1)]
         [TestCase(10)]
         [TestCase(50)]
+        [FlakyTest]
         public void TestNoDecoupledDrift(int updateRate)
         {
             var stopwatch = new StopwatchClock();

osu.Framework.Tests/Clocks/InterpolatingFramedClockTest.cs

@@ -254,6 +254,7 @@ namespace osu.Framework.Tests.Clocks
         [TestCase(1)]
         [TestCase(10)]
         [TestCase(50)]
+        [FlakyTest]
         public void TestNoInterpolationDrift(int updateRate)
         {
             var stopwatch = new StopwatchClock();

osu.Framework.Tests/Graphics/TripleBufferTest.cs

@@ -108,6 +108,7 @@ namespace osu.Framework.Tests.Graphics
         }
 
         [Test]
+        [FlakyTest]
         public void TestReadSaturated()
         {
             var tripleBuffer = new TripleBuffer<TestObject>();