Change cancellation test to not depend on how AwaitTask works (#7467)

* Change cancellation test to not depend on how AwaitTask works

This test is to check that when an Async is started as a Task (via
StartAsTask) that when cancelled the Task isn't immediately marked as
cancelled but instead waits for the underlying Async to finish (normally
via cancellation)

* Add test that AwaitTask ignore async cancellation

tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncType.fs

@@ -145,24 +145,26 @@ type AsyncType() =
     [<Test>]
     member this.StartAsTaskCancellation () =
         let cts = new CancellationTokenSource()
-        let tcs = TaskCompletionSource<unit>()
+        let mutable spinloop = true
+        let doSpinloop () = while spinloop do ()
         let a = async {
             cts.CancelAfter (100)
-            do! tcs.Task |> Async.AwaitTask }
+            doSpinloop()
+        }
 #if !NET46
         let t : Task<unit> =
 #else
         use t : Task<unit> =
 #endif
             Async.StartAsTask(a, cancellationToken = cts.Token)
 
-        // Should not finish
+        // Should not finish, we don't eagerly mark the task done just because it's been signaled to cancel.
         try
             let result = t.Wait(300)
             Assert.IsFalse (result)
-        with :? AggregateException -> Assert.Fail "Task should not finish, jet"
+        with :? AggregateException -> Assert.Fail "Task should not finish, yet"
 
-        tcs.SetCanceled()
+        spinloop <- false
 
         try
             this.WaitASec t
@@ -172,6 +174,31 @@ type AsyncType() =
             | _ -> reraise()
         Assert.IsTrue (t.IsCompleted, "Task is not completed")
 
+    [<Test>]
+    member this.``AwaitTask ignores Async cancellation`` () =
+        let cts = new CancellationTokenSource()
+        let tcs = new TaskCompletionSource<unit>()
+        let innerTcs = new TaskCompletionSource<unit>()
+        let a = innerTcs.Task |> Async.AwaitTask
+
+        Async.StartWithContinuations(a, tcs.SetResult, tcs.SetException, ignore >> tcs.SetCanceled, cts.Token)
+
+        cts.CancelAfter(100)
+        try
+            let result = tcs.Task.Wait(300)
+            Assert.IsFalse (result)
+        with :? AggregateException -> Assert.Fail "Should not finish, yet"
+
+        innerTcs.SetResult ()
+
+        try
+            this.WaitASec tcs.Task
+        with :? AggregateException as a ->
+            match a.InnerException with
+            | :? TaskCanceledException -> ()
+            | _ -> reraise()
+        Assert.IsTrue (tcs.Task.IsCompleted, "Task is not completed")
+
     [<Test>]
     member this.StartTask () =
         let s = "Hello tasks!"