Add next handler for Observables' tests

[dingyuchen]

Summary

This fixes #607, where not supplying the next handler lets the test pass automatically when the desired behavior is for an operator to throw an error.

Proposed Commit Message

Tests: Some Subscribers are missing its next handler

Let's add next handlers where appropriate so that tests
fail when there is undesired behavior.

[dingyuchen]

Before I make this PR ready for review @ptvrajsk could you help me take a look at whether I caught all the instances of missing next handlers?

I did a search on all files that matches expect(err) and I mainly only found tests in session.model.spec, of which most testcases are fixed. If you are aware of any other places there might be bugs do let me know! Thanks

[ptvrajsk]

Sure @dingyuchen I'll look through them 👍

[ptvrajsk]

I checked through my IDE as well and I don't think you missed out on any. Nice job! Just some minor parts to discuss.

[ptvrajsk]

LGTM 👍