Too-big server-side root metadata

[phenaproxima]

Description: This fixture does not need any targets, but it should have server-side and client-side metadata. There should be a second version of the server-side metadata, and its root metadata should just be a garbage file of 120KB or more of random characters.

Procedure: Generate a simple fixture. No need to add targets to it. Publish it once, with the client, and then publish it again, without the client. Replace the second version's root metadata with a file filled with randomly-generated text, and make it unreasonably large (at least 120 KB).

Expected Outcome: The TUF client should fail trying to fetch the new root metadata, because it's too big.