Fix test secret key to meet 64-byte minimum required by HS512

The firebase/php-jwt library requires HMAC keys to be at least as long
as the hash output (512 bits for HS512). Updated the test secret to a
base64-encoded 64-byte key to satisfy this requirement.

Author: byjg

tests/JwtSessionTest.php

@@ -30,7 +30,7 @@ class JwtSessionTest extends TestCase
     protected function setUp(): void
     {
         $this->sessionConfig = (new SessionConfig('example.com'))
-            ->withSecret('secretKey');
+            ->withSecret('c2VjcmV0S2V5X3RoaXNfaXNfYV92ZXJ5X2xvbmdfc2VjcmV0X2tleV90aGF0X2lzXzY0X2J5dGVzX2xvbmchIQ==');
 
         $this->object = new JwtSession($this->sessionConfig);
     }