No trusted root metadata available on the client side

[phenaproxima]

Description: This fixture should contain only a directory of trusted client-side metadata. That directory should be missing root metadata.

Procedure: Initialize a fixture; there's no need to add any targets. Publish it with the client, then delete the server directory, and delete the root file from the client directory.

Expected Outcome: The TUF client should fail because it cannot load the trusted root metadata.