Fix flaky GC-based tests in AsyncLazyTests

ValueFactoryReleasedAfterExecution and AsyncPumpReleasedAfterExecution were intermittently failing in CI because GC.Collect() alone does not guarantee collection of objects tied to async state machines. Replace the non-deterministic collection loop with a single Task.Yield() to release async state machine references, followed by forced blocking full GC with GC.WaitForPendingFinalizers() to ensure deterministic cleanup.

Author: AArnott

test/Microsoft.VisualStudio.Threading.Tests/AsyncLazyTests.cs

@@ -153,10 +153,11 @@ public async Task ValueFactoryReleasedAfterExecution()
     {
         WeakReference collectible = await this.ValueFactoryReleasedAfterExecution_Helper();
 
+        await Task.Yield();
         for (int i = 0; i < 3; i++)
         {
-            await Task.Yield();
-            GC.Collect();
+            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, blocking: true);
+            GC.WaitForPendingFinalizers();
         }
 
         Assert.False(collectible.IsAlive);
@@ -167,10 +168,11 @@ public async Task AsyncPumpReleasedAfterExecution(bool throwInValueFactory)
     {
         WeakReference collectible = await this.AsyncPumpReleasedAfterExecution_Helper(throwInValueFactory);
 
+        await Task.Yield();
         for (int i = 0; i < 3; i++)
         {
-            await Task.Yield();
-            GC.Collect();
+            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, blocking: true);
+            GC.WaitForPendingFinalizers();
         }
 
         Assert.False(collectible.IsAlive);