[Key Vault] Re-enable role assignment and definition tests in playback

[mccoyp]

As part of #35425, test_role_definitions and test_role_assignment tests were marked as live-only. The tests began failing in playback after recording with the latest test proxy version because the randomly-generated UUIDs used for role resources weren't being sanitized as expected in new recordings.

Instead of using sanitizers to record these UUIDs as some-uuid, we could instead use the variables API to store random UUIDs in recordings. However, the fact that tests are no longer able to sanitize in the same way they used to is unexpected and should be looked into.