fixed regression in the Finder component (it was possible to use it without using exec before, closes symfony#6357)

Author: fabpot

src/Symfony/Component/Finder/Shell/Shell.php

@@ -52,7 +52,11 @@ public function testCommand($command)
  {
  if (self::TYPE_WINDOWS === $this->type) {
  // todo: find a way to test if windows command exists
- return true;
+ return false;
+ }
+
+ if (!function_exists('exec')) {
+ return false;
  }
 
  // todo: find a better way (command could not be available)