Add a negative test for inactive validator for withdrawal request

[terencechain]

I noticed that by commenting out the inactive check for process_withdrawal_request, the test still passes. This PR adds a test case to ensure an inactive validator doesn't pass. It's my first time adding a spec test, so it may be off. Note to the reviewer: feel free to push any changes to my PR as needed

    # Verify the validator is active
    if not is_active_validator(validator, get_current_epoch(state)):
        return