[feature/no-ref/separate-rvo2] Add better unit tests and integration tests

[tashakim]

Existing tests are not comprehensive, and does not describe expected behavior well. Hence the code is very brittle.

Additional tests should be added for both (i) unit testing and (ii) integration testing (e2e simulation). Codebase particularly lacks (i) for internal methods.
Ideally, test-driven-development should be utilized going forward.