[tests] initialize _initialized (#4662)
[tests] initialize _initialized (#4662)

This commit initializes _initialized of Node before doing
initialization. Otherwise, we will have an AttributeError in
desctructor when initialization fails.
chshu/openthread,srickardti/openthread,lanyuwen/openthread,srickardti/openthread,chshu/openthread,jwhui/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,chshu/openthread,librasungirl/openthread,bukepo/openthread,openthread/openthread,librasungirl/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,chshu/openthread,abtink/openthread,bukepo/openthread,lanyuwen/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,abtink/openthread,bukepo/openthread,librasungirl/openthread,lanyuwen/openthread,jwhui/openthread,openthread/openthread,chshu/openthread