Fix broken TokenGeneratorTest::testIdTokenGenerationWithoutDpopKey UnitTest

[Potherca]

Looks like a Class mock needs to be replaced with an Interface mock.

Class "Lcobucci\JWT\Signer\Key\InMemory" is declared "final" and cannot be doubled

PHPUnit 9.6.13 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.0.30 with PCOV 1.0.11
Configuration: /home/runner/work/php-solid-auth/php-solid-auth/phpunit.xml.dist

............................E...............SSS..S.               51 / 51 (100%)

Time: 00:00.327, Memory: 22.00 MB

There was 1 error:

1) Pdsinterop\Solid\Auth\TokenGeneratorTest::testIdTokenGenerationWithoutDpopKey
PHPUnit\Framework\MockObject\ClassIsFinalException: Class "Lcobucci\JWT\Signer\Key\InMemory" is declared "final" and cannot be doubled

/home/runner/work/php-solid-auth/php-solid-auth/src/TokenGenerator.php:211
/home/runner/work/php-solid-auth/php-solid-auth/src/TokenGenerator.php:94
/home/runner/work/php-solid-auth/php-solid-auth/tests/unit/TokenGeneratorTest.php:313