1) Tests for "end game from user exit" functionality 2) bug fixes

#### What's this PR do?
This PR: 
1. Sets up tests for the "end game from user exit" functionality of competitive story games. Creates Game 1, then invites a player from Game 1 to Game 2, checks the state of both gameDocs and players from both games. 
2. Fixes a bug in the `_endGameFromPlayerExit` function. 

#### Where should the reviewer start?
Check out the `test/EndGameFromUserExit.js` function, then the bug fix in the `SGCompetitiveStoryController.js`. 

#### How should this be manually tested?
Run `npm test'! 

#### What are the relevant tickets?
Closes #303. 

That's what I thought initially, and then I realized that nearly all the helper functions in `testHelperFunctions.js` closed over a reference to `gameId`. 

Creating two gameId variables would require 1) rewriting the functions in `testHelperFunctions.js` to accept a gameId as an argument, and then 2) refactoring all the test files to use this new gameId convention. 

I actually began with this approach, ran into scoping complexities, and went ahead with the easier, hackier solution instead. :confused: Definitely happy to try to implement this, though. Or perhaps a good refactoring session could happen in the future, when we add more tests.