Thread: add multi-device test

This commit adds a basic multi-device test for Thread network.
Two nodes are used. One creates a network and the test verifies
the other one can discover it by Thread scan.

Test: presubmit
Bug: 329188649
Change-Id: Ice03015838bbf6fe836fac48c659a97850a89132
diff --git a/thread/TEST_MAPPING b/thread/TEST_MAPPING
index ebbb9af..34d67bb 100644
--- a/thread/TEST_MAPPING
+++ b/thread/TEST_MAPPING
@@ -11,5 +11,8 @@
     }
   ],
   "postsubmit": [
+    {
+      "name": "ThreadNetworkMultiDeviceTests"
+    }
   ]
 }