node/approval-voting: Continue to migrate tests to subsystem tests (#3471)

* node/approval-voting: Continue to migrate tests to subsystem tests

* node/approval-voting: Continue to implement subsystem tests.

* Add more tests

* node/approval-voting: Add more tests

* node/approval-voting: Difficuly determining the proper test scenario for threshold tests

* node/approval-voting: Introduce should_trigger_assignment tests

* node/approval-voting: Finalize threshold tests

* node/approval-voting: Address Feedback and add comments

* node/approval-voting: Tidy up approval tests

* Tidy up tests

* Fix rustfmt