This commit also updates the key loader helper in
test_updater_root_rotation_integration.py to load
the new keys too.
The keys were created (at the root of the repository) like
so:
```
from tuf import repository_tool
repository_tool.generate_and_write_ed25519_keypair(
"tests/repository_data/keystore/root_key2", "password")
repository_tool.generate_and_write_ed25519_keypair(
"tests/repository_data/keystore/root_key3", "password")
```
Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>