Add logging to sporadically failing ExecutionContext flow test (#28484)

Author: stephentoub

src/System.Net.Sockets/tests/FunctionalTests/ExecutionContextFlowTest.netcoreapp.cs

@@ -4,6 +4,7 @@
 
 using System.IO;
 using System.Runtime.CompilerServices;
+using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using Xunit;
@@ -26,8 +27,13 @@ public Task ExecutionContext_FlowsOnlyOnceAcrossAsyncOperations()
                     client.Connect(listener.LocalEndPoint);
                     using (Socket server = listener.Accept())
                     {
+                        var stackLog = new StringBuilder();
                         int executionContextChanges = 0;
-                        var asyncLocal = new AsyncLocal<int>(_ => executionContextChanges++);
+                        var asyncLocal = new AsyncLocal<int>(_ =>
+                        {
+                            executionContextChanges++;
+                            stackLog.AppendLine($"#{executionContextChanges}: {Environment.StackTrace}");
+                        });
                         Assert.Equal(0, executionContextChanges);
 
                         int numAwaits = 20;
@@ -44,7 +50,14 @@ public Task ExecutionContext_FlowsOnlyOnceAcrossAsyncOperations()
 
                         // This doesn't count EC changes where EC.Run is passed the same context
                         // as is current, but it's the best we can track via public API.
-                        Assert.InRange(executionContextChanges, 1, numAwaits * 3); // at most: 1 / AsyncLocal change + 1 / suspend + 1 / resume
+                        try
+                        {
+                            Assert.InRange(executionContextChanges, 1, numAwaits * 3); // at most: 1 / AsyncLocal change + 1 / suspend + 1 / resume
+                        }
+                        catch (Exception e)
+                        {
+                            throw new Exception($"{nameof(executionContextChanges)} == {executionContextChanges} with log: {stackLog.ToString()}", e);
+                        }
                     }
                 }
             });