Add test assembly definition for SerializeReferenceExtensions

Introduced a new Tests folder with an assembly definition file for unit testing MackySoft.SerializeReferenceExtensions in the Unity Editor. This setup enables integration with Unity's Test Runner and includes references to required assemblies.

Author: mackysoft

Assets/MackySoft/MackySoft.SerializeReferenceExtensions/Tests.meta

@@ -0,0 +1,25 @@
+{
+    "name": "Tests",
+    "rootNamespace": "",
+    "references": [
+        "UnityEngine.TestRunner",
+        "UnityEditor.TestRunner",
+        "MackySoft.SerializeReferenceExtensions",
+        "MackySoft.SerializeReferenceExtensions.Editor"
+    ],
+    "includePlatforms": [
+        "Editor"
+    ],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": true,
+    "precompiledReferences": [
+        "nunit.framework.dll"
+    ],
+    "autoReferenced": false,
+    "defineConstraints": [
+        "UNITY_INCLUDE_TESTS"
+    ],
+    "versionDefines": [],
+    "noEngineReferences": false
+}