Bump phpunit XSD version to 5.2

Some attributes being used in the phpunit configuration files, namely
failOnRisky and failOnWarning were introduced in phpunit 5.2.0. The
Composer configuration shows that tests should run with old versions of
phpunit, but phpunit only validates the configuration against the XSD
since phpunit 7.2.0.
These changes can be tested as follows:

wget http://schema.phpunit.de/5.2/phpunit.xsd
xargs xmllint --schema phpunit.xsd  1>/dev/null
find src -name phpunit.xml.dist| xargs xmllint --schema phpunit.xsd  1>/dev/null

See sebastianbergmann/phpunit@7e06a82
See https://github.com/symfony/symfony/blob/46e3745a03e199e64cc0fcf3284a96b5a25dcee9/composer.json#L98

phpunit.xml.dist

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/4.1/phpunit.xsd"
+         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/5.2/phpunit.xsd"
          backupGlobals="false"
          colors="true"
          bootstrap="vendor/autoload.php"