Checkstyle output needs to HTML-escape messages

[gmazzap]

The simplex example, let's take a file containing:

<?php

$x = foo";

If we run PHP Parallel Lint with the --checkstyle flag we get (formatted for readability):

<?xml version="1.0" encoding="UTF-8"?>
<checkstyle>
  <file name="example.php">
    <error
      line="3"
      severity="ERROR"
      message="Parse error: syntax error, unexpected '"' in ./inc/x.php on line 3"
      source="Syntax Error" />
    </file>
</checkstyle>

Which is broken XML.

Generally speaking, when generating checkstyle XML, PHP Parallel Lint should HTML-escape messages, via something like: htmlspecialchars($message, ENT_QUOTES, 'UTF-8'); here: https://github.com/php-parallel-lint/PHP-Parallel-Lint/blob/master/src/Output.php#L444