gh-1880 + tests

Closes #1880
It's surprising that these are the first tests for JSHint's "read from standard in" behavior, but I think we can take this opportunity to recognize that any future tests would have similar needs. What do you think about making a new "test suite" object--something like:

```js
  stdin: {
    setUp: function(done) {
      this.stdin = require('mock-stdin').stdin();
      done();
    },
    tearDown: function(done) {
      this.stdin.restore();
      done();
    },
    // your new tests here (with `stdin` replace with `this.stdin`)
  }
```

...this also saves you from the "STDIN" test name suffix (since the suite name will be prefixed on the tests in the Nodeunit report).