Rule: prefer output: null to assert that a test case is not autofixed

[not-an-aardvark]

Disallows invalid RuleTester test cases with the output the same as the code.

Invalid:

/* eslint eslint-plugin/prefer-output-null: error */

new RuleTester().run('foo', rule, {
  valid: [],
  invalid: [
    {
      code: 'foo',
      output: 'foo',
      errors: 1
    }
  ]
});

Valid:

/* eslint eslint-plugin/prefer-output-null: error */

new RuleTester().run('foo', rule, {
  valid: [],
  invalid: [
    {
      code: 'foo',
      output: null,
      errors: 1
    }
  ]
});

/* eslint eslint-plugin/prefer-output-null: error */

new RuleTester().run('foo', rule, {
  valid: [],
  invalid: [
    {
      code: 'foo',
      output: 'bar',
      errors: 1
    }
  ]
});