[Thread] add ThreadNetworkUnitTests to postsubmit

Change-Id: I475096305728ba5177d680d5d09b7ee3a89a8ed8
diff --git a/thread/TEST_MAPPING b/thread/TEST_MAPPING
index 3eaebfa..30aeca5 100644
--- a/thread/TEST_MAPPING
+++ b/thread/TEST_MAPPING
@@ -3,5 +3,10 @@
     {
       "name": "CtsThreadNetworkTestCases"
     }
+  ],
+  "postsubmit": [
+    {
+      "name": "ThreadNetworkUnitTests"
+    }
   ]
 }