Throw an exception on unit test assertion fail.

tests/Avalonia.UnitTests/ModuleInitializer.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace Avalonia.Base.UnitTests
+{
+    internal static class ModuleInitializer
+    {
+        [ModuleInitializer]
+        internal static void TestInit()
+        {
+            Trace.Listeners.Insert(0, new ThrowListener());
+        }
+
+        private class ThrowListener : TextWriterTraceListener
+        {
+            public override void Fail(string message)
+            {
+                throw new Exception("Assertion Failed. " + message);
+            }
+
+            public override void Fail(string message, string detailMessage)
+            {
+                throw new Exception("Assertion Failed. " + message + detailMessage);
+            }
+        }
+    }
+}