Add setup and teardown for tests

[aadharna]

We need to add setup and teardown functionality to our testing so that we can spin up a ray cluster run tests that require ray (e.g. validator tests) and then spin it down. This will fix code such as the MockSolver class as well as the ValidatorTests where there is now some ray remote calls outside of the POETManager.

This came up for the first time in #19 where we got around it by disabling one of the Validator tests.

[aadharna]

The folks over at ray pointed us at this for pytest: https://github.com/pynb-dag-runner/pynb-dag-runner/blob/development/workspace/pynb_dag_runner/conftest.py

ismail here mentions that one way to only run something once would be to add it to the init.py file. https://stackoverflow.com/questions/26884198/python-unit-tests-run-function-after-all-test

Here's setup and teardown examples from python docs: https://docs.python.org/3/library/unittest.html#setupclass-and-teardownclass. This would setup (ray.init) and teardown (ray.shutdown) at the file level.

[aadharna]

@rohin-dasari, you mentioned interest in switching to pytest. If you make that happen, you should probably look at the first link in the above comment this should let you spin up Ray in the tests only once if I understand it correctly.

The folks over at ray pointed us at this for pytest: https://github.com/pynb-dag-runner/pynb-dag-runner/blob/development/workspace/pynb_dag_runner/conftest.py