Parse attributeless <failure> tags correctly

[sujeet]

jest-junit generates failure tags that have no attributes, only inner text. For example: <failure>Failed!</failure>.

The xml2js library seems to produce different objects based on whether a tag has attributes. See the minimal example:

var parseString = require('xml2js').parseString;
var print = (err, result) => console.log(JSON.stringify(result, null, 2))
var parse = (xmlStr) => parseString(xmlStr, print)

parse('<a b="c">d</a>')
{
  "a": {
    "_": "d",
    "$": {
      "b": "c"
    }
  }
}

parse('<a>d</a>')
{
  "a": "d"
}

Notice how the second output above is not {"a": {"_": "d"}}

With this change, we take into account this difference while parsing the failure messages.