Either you open and close this session each of your testcase or make your invidual tests as part of procedures such that the setup and cleanup procedures can be called as part of your other tests too.
Sessions cannot be preserved across testcase runs. So you have to do as Anku suggested - open the session again in each testcase.
Why don't you open and close session s1 during in setup test case.Then in every testcase you can open and close session s1.In every test case you will have to put s1 in the session field in every command.
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