[Debug] fix compat with PHP 7.3

Author: nicolas-grekas

src/Symfony/Component/Debug/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".