adding test constraint to test asmdef (#639)

Author: fernando-cortez

Packages/com.unity.multiplayer.samples.coop/Utilities/Testing/com.unity.multiplayer.samples.utilities.testing.asmdef

@@ -12,7 +12,9 @@
         "nunit.framework.dll"
     ],
     "autoReferenced": true,
-    "defineConstraints": [],
+    "defineConstraints": [
+        "UNITY_INCLUDE_TESTS"
+    ],
     "versionDefines": [],
     "noEngineReferences": false
 }