[Security] Make fixture compatible with both 5.x and 6.x

Author: chalasr

Tests/EventListener/PasswordMigratingListenerTest.php

@@ -141,4 +141,6 @@ abstract public function loadUserByIdentifier(string $identifier): UserInterface
 abstract class TestPasswordAuthenticatedUser implements UserInterface, PasswordAuthenticatedUserInterface
 {
     abstract public function getPassword(): ?string;
+
+    abstract public function getSalt();
 }