test(runtime): keep cancellation observers alive (#10790)

Author: ReubenBond

test/Orleans.Runtime.Tests/CancellationTests/CancellationTokenTests.cs

@@ -97,6 +97,7 @@ public async Task GrainTaskCancellation(int delay)
         finally
         {
             fixture.GrainFactory.DeleteObjectReference<ILongRunningTaskObserver>(observerReference);
+            GC.KeepAlive(observer);
         }
     }
 
@@ -142,6 +143,7 @@ await Task.WhenAll(grains.Select((grain, index) =>
         finally
         {
             fixture.GrainFactory.DeleteObjectReference<ILongRunningTaskObserver>(observerReference);
+            GC.KeepAlive(observer);
         }
     }
 
@@ -195,6 +197,7 @@ await Task.WhenAll(grainTasks.Select(task =>
             }
 
             fixture.GrainFactory.DeleteObjectReference<ILongRunningTaskObserver>(observerReference);
+            GC.KeepAlive(observer);
         }
     }
 
@@ -476,6 +479,7 @@ public async Task InterleavingGrainTaskCancellation(int delay)
         finally
         {
             fixture.GrainFactory.DeleteObjectReference<ILongRunningTaskObserver>(observerReference);
+            GC.KeepAlive(observer);
         }
     }
 

test/Orleans.Runtime.Tests/CancellationTests/GrainCancellationTokenTests.cs

@@ -184,6 +184,7 @@ public async Task CancellationTokenCallbacksThrow_ExceptionShouldBePropagated()
             finally
             {
                 fixture.GrainFactory.DeleteObjectReference<ILongRunningTaskObserver>(observerReference);
+                GC.KeepAlive(observer);
             }
         }