[Automation] Add functions to create TCP server and create child process.

- Provide a util function createTCPServer() to create TCP & socket server. The server object should be released at the end of current process.
- Provide a util function spawnChildProcess() to spawn a child process by inputing an absolute path of the new process.
[Automation] Add test case "app-single-instance".
Add start_app test case
Add buff_from_string test case
Merge pull request #2708 from yejingfu/automation

More automation test case added.