ngclient: test fast-forward recovery

[jku]

I think we do not currently test successful rollback (aka fast-forward recovery) for timestamp, snapshot and targets:

• simulate a fast-forward attack: create repository with valid timestamp/snapshot/targets with version=MAX_VERSION (or something large anyway)
• update client so it sees the large versions
• rotate the affected key (timestamp or snapshot) and set metadata version=1
• update client, assert that new metadata is accepted by ngclient (even though the version number is lower than the local version)

[kairoaraujo]

Please, can you sign this issue for me?

[jku]

create repository with valid timestamp/snapshot/targets with version=MAX_VERSION

I'm not sure if there even is a practical MAX_VERSION in our implementation -- python seems to seamlessly upgrade from an int to some big int implementation... I think this is not relevant anyway: the point is that version is something large that we want to reinitialize to 1.

[jku]

I think we're done here! Thanks Kairo