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

zebra/zebra-rpc/src/methods/tests/snapshot.rs

Lines 54 to 63 in 824e520

	/// TODO:
	/// 1. Check a non-empty Sapling treestate.
	/// 2. Check an empty Orchard treestate at NU5 activation height.
	/// 3. Check a non-empty Orchard treestate.
	///
	/// To implement the todos above, we need to:
	///
	/// 1. Have a block containing Sapling note commitmnets in the state.
	/// 2. Activate NU5 at a height for which we have a block in the state.
	/// 3. Have a block containing Orchard note commitments in the state.

Depends on #8535.