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. 

Not that necessary, but I wanted it to be clear from the tests that we were inviting a player out of a game that was fully in session; that gameplay had already ensued. 

If the tests are easier to read and more parsimonious without gameplay, definitely happy to remove it.