Improve test class discovery

source/TestAdapter/Discover.cs

@@ -120,7 +120,7 @@ public static List<TestCase> ComposeTestCases(string sourceFile)
             AppDomain.CurrentDomain.Load(test.GetName());
 
             var typeCandidatesForTests = test.GetTypes()
-                                            .Where(x => x.IsClass);
+                                            .Where(Helper.IsTestClassCandidate);
 
             foreach (var typeCandidate in typeCandidatesForTests)
             {

source/TestFrameworkShared/Helper.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
+
 namespace nanoFramework.TestFramework
 {
     /// <summary>
@@ -10,6 +12,43 @@ public static class Helper
     {
         private delegate bool AnyDelegateType(object source);
 
+        /// <summary>
+        /// Checks whether a type can be considered for test discovery and execution.
+        /// </summary>
+        /// <param name="type">Type to inspect.</param>
+        /// <returns><see langword="true"/> when the type is a class and not an attribute type.</returns>
+        public static bool IsTestClassCandidate(Type type)
+        {
+            return type.IsClass && !IsAttributeType(type);
+        }
+
+        private static bool IsAttributeType(Type type)
+        {
+            var attributeFullName = typeof(Attribute).FullName;
+
+            Type current = type;
+            while (current != null)
+            {
+                if (current.FullName == attributeFullName)
+                {
+                    return true;
+                }
+                try
+                {
+                    current = current.BaseType;
+                }
+                catch
+                {
+                    // Base type assembly not resolvable in this reflection context;
+                    // conservatively treat as attribute to avoid calling GetCustomAttributes on it.
+                    return true;
+                }
+            }
+
+            return false;
+
+        }
+
         private static bool Any(this object[] array, AnyDelegateType predicate)
         {
             foreach (var item in array)

source/UnitTestLauncher/Program.cs

@@ -22,7 +22,7 @@ public static void Main()
 
             foreach (var type in allTypes)
             {
-                if (!type.IsClass)
+                if (!Helper.IsTestClassCandidate(type))
                 {
                     continue;
                 }