[tests] initialize _initialized (#4662)

This commit initializes _initialized of Node before doing
initialization. Otherwise, we will have an AttributeError in
desctructor when initialization fails.

tests/scripts/thread-cert/node.py

@@ -48,6 +48,7 @@ class Node:
         self.verbose = int(float(os.getenv('VERBOSE', 0)))
         self.node_type = os.getenv('NODE_TYPE', 'sim')
         self.env_version = os.getenv('THREAD_VERSION', '1.1')
+        self._initialized = False
 
         if version is not None:
             self.version = version