Throw when user tries to run a non-async test with a timeout

src/xunit.execution/Sdk/Frameworks/Runners/XunitTestInvoker.cs

@@ -116,6 +116,9 @@ protected override Task<decimal> InvokeTestMethodAsync(object testClassInstance)
 
         async Task<decimal> InvokeTimeoutTestMethodAsync(object testClassInstance)
         {
+            if (!AsyncUtility.IsAsync(TestMethod))
+                throw new TestTimeoutException();
+
             var baseTask = base.InvokeTestMethodAsync(testClassInstance);
             var resultTask = await Task.WhenAny(baseTask, Task.Delay(TestCase.Timeout));
 

src/xunit.execution/Sdk/Frameworks/TestTimeoutException.cs

@@ -9,7 +9,16 @@ namespace Xunit.Sdk
     public class TestTimeoutException : Exception
     {
         /// <summary>
-        /// Initializes a new instance of <see cref="TestTimeoutException"/>.
+        /// Initializes a new instance of the <see cref="TestTimeoutException"/> class, returning a
+        /// message indicating that the test method isn't compatible with timeout functionality.
+        /// </summary>
+        public TestTimeoutException()
+            : base("Tests marked with Timeout are only supported for async tests")
+        { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="TestTimeoutException"/> class, returning a
+        /// message indicating that the test method timed out.
         /// </summary>
         /// <param name="timeout">The timeout that was exceeded, in milliseconds</param>
         public TestTimeoutException(int timeout)

test/test.xunit.execution/Acceptance/Xunit2AcceptanceTests.cs

@@ -192,7 +192,7 @@ class ClassUnderTest
             public Task LongRunningTest() => Task.Delay(10000);
 
             [Fact(Timeout = 10000)]
-            public void ShortRunningTest() => Task.Delay(10);
+            public Task ShortRunningTest() => Task.Delay(10);
         }
     }