Added UNITY_TEST_FRAMEWORK_1_7_OR_NEWER guard and UnityCoreClrExplicitDisabledAttributeShim.cs to make attribute usage possible

Author: michalChrobot

com.unity.netcode.gameobjects/Tests/Runtime/Unity.Netcode.Runtime.Tests.asmdef

@@ -47,6 +47,11 @@
             "name": "Unity",
             "expression": "6000.1.0a1",
             "define": "HOSTNAME_RESOLUTION_AVAILABLE"
+        },
+        {
+            "name": "com.unity.test-framework",
+            "expression": "1.7.0",
+            "define": "UNITY_TEST_FRAMEWORK_1_7_OR_NEWER"
         }
     ],
     "noEngineReferences": false

com.unity.netcode.gameobjects/Tests/Runtime/UnityCoreClrExplicitDisabledAttributeShim.cs

@@ -0,0 +1,14 @@
+#if !UNITY_TEST_FRAMEWORK_1_7_OR_NEWER
+using System;
+
+namespace UnityEngine.TestTools
+{
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+    internal sealed class UnityCoreClrExplicitDisabledAttribute : Attribute
+    {
+        public UnityCoreClrExplicitDisabledAttribute(string jiraIssue, string reason = null)
+        {
+        }
+    }
+}
+#endif