Trusted root metadata is expired and no more recent version is available

[phenaproxima]

Description: This should be a simple fixture with server- and client-side metadata. The root metadata in both directories should have a date way in the past, to guarantee that it is expired and cannot be updated.

Procedure: TBD; I'm not clear yet on how to manually modify the expiration date of any given piece of metadata, beyond forcibly re-signing it.

Expected Outcome: The TUF client should fail because the most recent available root metadata is expired.