simplify SimulateTimeout by using SimulateException

Test/Flurl.Test/Http/TestingTests.cs

@@ -378,13 +378,23 @@ public async Task can_simulate_exception() {
 		[Test]
 	    public async Task can_simulate_timeout_with_exception_handled() {
 	        HttpTest.SimulateTimeout();
-	        var result = await "http://www.api.com"
-	            .ConfigureRequest(c => c.OnError = call => call.ExceptionHandled = true)
-	            .GetAsync();
-	        Assert.IsNull(result);
-	    }
+	        var exceptionCaught = false;
+
+	        var resp = await "http://api.com"
+		        .ConfigureRequest(c => c.OnError = call => {
+			        exceptionCaught = true;
+			        var ex = call.Exception as TaskCanceledException;
+			        Assert.NotNull(ex);
+			        Assert.IsInstanceOf<TimeoutException>(ex.InnerException);
+			        call.ExceptionHandled = true;
+		        })
+		        .GetAsync();
+
+	        Assert.IsNull(resp);
+	        Assert.IsTrue(exceptionCaught);
+		}
 
-	    [Test]
+		[Test]
 		public async Task can_fake_headers() {
 			HttpTest.RespondWith(headers: new { h1 = "foo" });
 

src/Flurl.Http/Testing/FakeHttpMessageHandler.cs

@@ -35,12 +35,7 @@ protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage reques
 							Content = new StringContent("")
 						};
 
-						var tcs = new TaskCompletionSource<HttpResponseMessage>();
-						if (resp is TimeoutResponseMessage)
-							tcs.SetCanceled();
-						else
-							tcs.SetResult(resp);
-						return tcs.Task;
+						return Task.FromResult(resp);
 					}
 				}
 			}

src/Flurl.Http/Testing/HttpTestSetup.cs

@@ -4,6 +4,7 @@
 using System.Net;
 using System.Net.Http;
 using System.Threading;
+using System.Threading.Tasks;
 using Flurl.Http.Configuration;
 using Flurl.Http.Content;
 using Flurl.Util;
@@ -105,16 +106,13 @@ public HttpTestSetup RespondWith(Func<HttpContent> buildContent = null, int stat
 		/// <summary>
 		/// Adds a simulated timeout response to the response queue.
 		/// </summary>
-		public HttpTestSetup SimulateTimeout() {
-			_responses.Add(() => new TimeoutResponseMessage());
-			return this;
-		}
+		public HttpTestSetup SimulateTimeout() =>
+			SimulateException(new TaskCanceledException(null, new TimeoutException())); // inner exception covers the .net5+ case https://stackoverflow.com/a/65989456/62600
 
 		/// <summary>
-		/// Add the throwing of an exception to the response queue
+		/// Adds the throwing of an exception to the response queue.
 		/// </summary>
-		/// <param name="exception"></param>
-		/// <returns></returns>
+		/// <param name="exception">The exception to throw when the call is simulated.</param>
 		public HttpTestSetup SimulateException(Exception exception) {
 			_responses.Add(() => throw exception);
 			return this;