Fix linting issues

[TheBookKnight]

When I ran npm run lint, I get all these errors

% npm run lint

> [email protected] lint
> eslint .


/Users/macpro/Documents/GitHub/choose-your-adventure/__tests__/chapter.test.js
   8:1   error  'jest' is not defined      no-undef
  10:21  error  'jest' is not defined      no-undef
  11:21  error  'jest' is not defined      no-undef
  12:27  error  'jest' is not defined      no-undef
  17:1   error  'describe' is not defined  no-undef
  18:5   error  'test' is not defined      no-undef
  28:9   error  'expect' is not defined    no-undef
  29:9   error  'expect' is not defined    no-undef
  30:9   error  'expect' is not defined    no-undef
  33:5   error  'test' is not defined      no-undef
  37:15  error  'expect' is not defined    no-undef

/Users/macpro/Documents/GitHub/choose-your-adventure/__tests__/scene.test.js
   7:1    error  'jest' is not defined      no-undef
   9:21   error  'jest' is not defined      no-undef
  10:21   error  'jest' is not defined      no-undef
  11:27   error  'jest' is not defined      no-undef
  16:1    error  'describe' is not defined  no-undef
  17:5    error  'test' is not defined      no-undef
  20:9    error  'expect' is not defined    no-undef
  20:73   error  'expect' is not defined    no-undef
  23:5    error  'test' is not defined      no-undef
  26:9    error  'expect' is not defined    no-undef
  26:110  error  'expect' is not defined    no-undef
  27:9    error  'expect' is not defined    no-undef
  27:62   error  'expect' is not defined    no-undef
  28:9    error  'expect' is not defined    no-undef
  31:5    error  'test' is not defined      no-undef
  34:9    error  'expect' is not defined    no-undef
  34:111  error  'expect' is not defined    no-undef
  35:9    error  'expect' is not defined    no-undef
  35:90   error  'expect' is not defined    no-undef
  36:9    error  'expect' is not defined    no-undef
  39:5    error  'test' is not defined      no-undef
  41:15   error  'expect' is not defined    no-undef

/Users/macpro/Documents/GitHub/choose-your-adventure/src/game-engine.js
  10:17  error  Unexpected negating the left operand of 'instanceof' operator  no-unsafe-negation

✖ 34 problems (34 errors, 0 warnings)