fix(symfony): ensure the kernel is booted before using KernelBrowser::loginUser() (#7446)

Author: ttskch

src/Symfony/Bundle/Test/Client.php

@@ -236,6 +236,8 @@ private static function extractHeaders(array $options): array
 
     public function loginUser(UserInterface $user, string $firewallContext = 'main'): self
     {
+        $this->kernelBrowser->getKernel()->boot();
+
         $this->kernelBrowser->loginUser($user, $firewallContext);
 
         return $this;