[DevEx]: Move all testing-specific endpoints into the TestController

[itsalaidbacklife]

Improvement Summary

Currently some of the testing-specific endpoints (the ones only allowed during development or staging) are in the TestController, and some are in the api/controllers/game folder. We should consolidate all of them into the TestController for consistency and simplicity.

Detailed Description

We should take all of the backend actions/endpoints which have the 'developmentOrStagingOnly' policy applied to them in config/policies.js and move them into api/controllers/TestController. While we are at it, we should update the urls to be more consistent and remove unnecessary testing-specific endpoints. For the unnecessary endpoints we should delete the files in api/controllers/game/<unnecessary_endpoint_name>.js, the policy mapping in config/policies.js, and the route mapping in config/routes.js

In particular:

1. game/stackDeck is no longer used and can be deleted
2. game/clear is no longer used and can be deleted
3. /game/loadGameFixture should be remapped to /test/loadGameFixture in config/routes.js and the corresponding endpoint file should be moved from api/controllers/game/load-fixturetoapi/test/load-game-fixture.js`