test: fix flaky AvaloniaScheduler zero-delay test

Replace the fixed Thread.Sleep(50) + bool flag with a TaskCompletionSource
awaited via WaitAsync(10s). When the scheduler runs off the dispatcher's UI
thread it posts the action at Background priority; on loaded macOS CI runners
50ms was occasionally too short before the assertion, causing intermittent
failures. The action now completes instantly on the synchronous path and gets
ample time (failing loudly via timeout) on the posted path.

Author: glennawatson

src/tests/ReactiveUI.Avalonia.Tests/AvaloniaUIThreadTestsMain.cs

@@ -81,17 +81,19 @@ public async Task AvaloniaScheduler_Now_ReturnsCurrentTime()
     public async Task AvaloniaScheduler_Schedule_WithZeroDelay_ExecutesAction()
     {
         var scheduler = AvaloniaScheduler.Instance;
-        var actionExecuted = false;
+        var tcs = new TaskCompletionSource();
 
         var disposable = scheduler.Schedule("test", TimeSpan.Zero, (s, state) =>
         {
-            actionExecuted = true;
+            tcs.TrySetResult();
             return Disposable.Empty;
         });
 
-        Thread.Sleep(50);
-
-        await Assert.That(actionExecuted).IsTrue();
+        // When invoked on the UI thread the action runs synchronously; otherwise it is posted to the
+        // dispatcher. Await its completion with a timeout rather than a fixed sleep so the test is not
+        // sensitive to dispatcher scheduling latency on busy CI machines (the previous fixed 50ms sleep
+        // was occasionally too short on loaded macOS runners, causing intermittent failures).
+        await tcs.Task.WaitAsync(TimeSpan.FromSeconds(10));
         await Assert.That(disposable).IsNotNull();
     }