tests: ngclient non-root key rotations

[jku]

We probably should have a few key rotation tests for non-root metadata as well. There aren't as many different scenarios as with root since "key rotation" just means verifying a delegate with threshold in this case (and we already test verify_delegate() itself pretty well) but let's add something to test_updater_key_rotations.py, for all non-root keys in root metadata and for a delegated targets key in a targets metadata as well.

See also #1607