[tests]add case for multiple instances

**PLEASE NOE MERGE THIS**

How to use:
- place it under topdir of seafile-server binary tarballs
- start up seafile and seahub
- make sure you have a account which username is "test@test.com" and passw is "test"
- initialize test data via ``./test-multiple-clients.sh init``
- start test via ``./test-multiple-clients.sh``
- customize it if you wish to