Retry flaky test and add some logging (#22953)

Author: christothes

sdk/core/Azure.Core/tests/BearerTokenAuthenticationPolicyTests.cs

@@ -676,6 +676,7 @@ void UnobservedTaskExceptionHandler(object sender, UnobservedTaskExceptionEventA
         }
 
         [Test]
+        [Retry(3)] //https://github.com/Azure/azure-sdk-for-net/issues/21005
         public async Task BearerTokenAuthenticationPolicy_BackgroundRefreshCancelledAndLogs()
         {
             var requestMre = new ManualResetEventSlim(true);
@@ -687,6 +688,7 @@ public async Task BearerTokenAuthenticationPolicy_BackgroundRefreshCancelledAndL
             string msg = "fail to refresh";
             var credential = new BearerTokenAuthenticationPolicyTests.TokenCredentialStub((r, c) =>
                 {
+                    TestContext.WriteLine($"Start TokenCredentialStub: requestCount: {requestCount}");
                     if (Interlocked.Increment(ref requestCount) > 1)
                     {
                         Task.Delay(100).GetAwaiter().GetResult();
@@ -697,12 +699,14 @@ public async Task BearerTokenAuthenticationPolicy_BackgroundRefreshCancelledAndL
                     requestMre.Reset();
 
                     expires.TryDequeue(out var token);
+                    TestContext.WriteLine($"End TokenCredentialStub: callCount: {requestCount}");
                     return new AccessToken(Guid.NewGuid().ToString(), token);
                 },
                 IsAsync);
 
             AzureEventSourceListener listener = new((args, text) =>
             {
+                TestContext.WriteLine(text);
                 if (args.EventName == "BackgroundRefreshFailed" && text.Contains(msg))
                 {
                     logged = true;