Add tests for session error and close

[ramya-rao-a]

Description

Brief description of the changes made in the PR. This helps in making better changelog

• Add tests for session_close and session_error events based on the patterns used in connection.spec.ts file

In #34, we ensured that all listeners on a link/session are removed when the link/session is closed.
Due to this, the test for session_close was failing as the listener was being removed before session.close() completed.
So, this PR also includes

• move the removing of listeners only after the close() operation is complete instead of doing so in the very beginning
• tests to ensure that we have no listeners attached after the close() operation completes

Reference to any github issues

• [Ops] Add tests to project #58

[ramya-rao-a]

cc @richardpark-msft

[chradek]

Looks good to me!