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 7e06a82806
See 46e3745a03/composer.json (L98)

Debug.php

@@ -45,7 +45,7 @@ class Debug
         if (!\in_array(\PHP_SAPI, array('cli', 'phpdbg'), true)) {
             ini_set('display_errors', 0);
             ExceptionHandler::register();
-        } elseif ($displayErrors && (!ini_get('log_errors') || ini_get('error_log'))) {
+        } elseif ($displayErrors && (!filter_var(ini_get('log_errors'), FILTER_VALIDATE_BOOLEAN) || ini_get('error_log'))) {
             // CLI - display errors only if they're not already logged to STDERR
             ini_set('display_errors', 1);
         }

ErrorHandler.php

@@ -115,7 +115,7 @@ class ErrorHandler
     /**
      * Registers the error handler.
      *
-     * @param self|null|int $handler The handler to register, or @deprecated (since version 2.6, to be removed in 3.0) bit field of thrown levels
+     * @param self|int|null $handler The handler to register, or @deprecated (since version 2.6, to be removed in 3.0) bit field of thrown levels
      * @param bool          $replace Whether to replace or not any existing handler
      *
      * @return self The registered error handler

Tests/phpt/decorate_exception_hander.phpt

@@ -1,5 +1,7 @@
 --TEST--
 Test catching fatal errors when handlers are nested
+--INI--
+display_errors=0
 --FILE--
 <?php
 
@@ -24,7 +26,6 @@ if (true) {
 
 ?>
 --EXPECTF--
-Fatal error: Class 'Symfony\Component\Debug\missing' not found in %s on line %d
 object(Symfony\Component\Debug\Exception\ClassNotFoundException)#%d (8) {
   ["message":protected]=>
   string(131) "Attempted to load class "missing" from namespace "Symfony\Component\Debug".

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"