Add `JWT::resetTime()` && `JWT::maxLeeway()` to simplify unit tests

[cosmastech]

When unit testing, we have fixtures for valid JWTs. In tests, we often have to setUp/tearDown where we re/set these values. This is designed to make that easier.

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[bshaffer]

Thanks for your contribution!

I don't generally like exposing functions like this which are 1) testing only and 2) already possible with roughly the same amount of code.