fix(mutator/id_token): fix token expiration error in tests

[kaorimatz]

Related issue

CI test for #389: https://circleci.com/gh/ory/oathkeeper/3986

Proposed changes

• Configure the TTL only if it is greater than 0 (the zero value of time.Duration) to avoid "Token is expired" error.
• Fix an assert for the exp claim. The default TTL is 15 minutes.

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

Further comments

[aeneasr]

Thank you!