Merge branch '5.0' into 5.1

* 5.0:
  Fix abstract method name in PHP doc block
  Various cleanups
  [HttpClient] fix issues in tests
  Fixes sprintf(): Too few arguments in form transformer
  [Console] Fix QuestionHelper::disableStty()
  [Validator] Use Mime component to determine mime type for file validator
  validate subforms in all validation groups
  Update Hungarian translations
  Add meaningful message when Process is not installed (ProcessHelper)
  [PropertyAccess] Fix TypeError parsing again.
  [TwigBridge] fix fallback html-to-txt body converter
  [Security/Http] fix merge
  [ErrorHandler] fix setting $trace to null in FatalError
  [Form] add missing Czech validators translation
  [Validator] add missing Czech translations
  never directly validate Existence (Required/Optional) constraints

Author: nicolas-grekas

Test/TestHttpServer.php

@@ -16,22 +16,23 @@
 
 class TestHttpServer
 {
-    private static $started;
+    private static $process;
 
     public static function start()
     {
-        if (self::$started) {
-            return;
+        if (self::$process) {
+            self::$process->stop();
         }
 
         $finder = new PhpExecutableFinder();
         $process = new Process(array_merge([$finder->find(false)], $finder->findArguments(), ['-dopcache.enable=0', '-dvariables_order=EGPCS', '-S', '127.0.0.1:8057']));
         $process->setWorkingDirectory(__DIR__.'/Fixtures/web');
         $process->start();
 
-        register_shutdown_function([$process, 'stop']);
-        sleep('\\' === \DIRECTORY_SEPARATOR ? 10 : 1);
+        do {
+            usleep(50000);
+        } while (!@fopen('http://127.0.0.1:8057/', 'r'));
 
-        self::$started = true;
+        self::$process = $process;
     }
 }