_getFile(instance) {
    var filePath = this._getAbsolutePath(instance.filename);
    return `<file path="${filePath}" line="${instance.start.line}"/>\n`;
  }
Returns an XML string containing the path to the file in which the instance
is located, as well as its starting line. Absolute paths are required for
Jenkins.

@param   {object} instance
@returns {string}