Test non-empty treestates in the `z_get_treestate` RPC

[upbqdn]

We're currently testing only non-existing and empty treestates in the z_get_treestate RPC. We should also test:

1. Non-empty Sapling treestates.
2. The empty Orchard treestate.
3. Non-empty Orchard treestates.

To close this issue, address the following todos https://github.com/ZcashFoundation/zebra/blob/824e5200e22438c322914cd776f0b89808a98e97/zebra-rpc/src/methods/tests/snapshot.rs#L54-L63

Depends on #8535.