adding tests for token Creation

[Dhouib-Mohamed]

1. Removing The default tests
2. Adding token.service.spec.ts containing :

• Creating testing module and from it creating a testing token service
• First Test to check if the service is defined
• Second test to check if encode function works and return a defined token
• Third test to check if the decode function works correctly with a correct token
• Fourth Test to check the error message on decoding invalid token
• Fifth Test to check the error message on decoding an expired token

3. Checking the tests coverage and validity using the commands defined in the package.json file