RuleTester prints `\u000a` instead of newline

Since eslint 6, rule tester prints newline in codes as `\u000a`. It is hard to read for plugin developers. 

#### Code to repoduce
```
npx mocha index.js
```

```js
// index.js

const {RuleTester} = require('eslint');

const someRule = {
  create() {
    return {};
  },
};

const ruleTester = new RuleTester();

ruleTester.run('some-rule', someRule, {
  valid: [],
  invalid: [
    {
      code: `
        var a;
        var b;
      `,
      errors: [{message: 'foooooooooooo'}],
    },
  ],
});
```
#### Actual output
Note the lines with `\u000a`.
```
  0 passing (46ms)
  1 failing

  1) some-rule
       invalid
         \u000a        var a;\u000a        var b;\u000a      :

      AssertionError [ERR_ASSERTION]: Should have 1 error but had 0: []
      + expected - actual

      -0
      +1
```

#### Expected ouput
This is the behavior with `npm i eslint@5`.
```
  1) some-rule
       invalid
         
        var a;
        var b;
      :

      AssertionError [ERR_ASSERTION]: Should have 1 error but had 0: []
      + expected - actual

      -0
      +1
```



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Uh oh!

RuleTester prints `\u000a` instead of newline #12027

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Uh oh!

RuleTester prints \u000a instead of newline #12027

Description

Code to repoduce

Actual output

Expected ouput

Metadata

Metadata

Assignees

Labels

Type

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Development

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RuleTester prints `\u000a` instead of newline #12027