Fix PHPUnit test that fails on Windows

The assertion "Test appending with set()" in
vendor/Joomla/Database/Tests/QueryTest.php:1362 fails, because it's using
PHP_EOL in the test, and \n in the implementation. On Windows, PHP_EOL is
set to \r\n and so the test passes on Travis CI, but fails on Windows.

- Change implementation, use PHP_EOL instead of \n