Add a TODO for parallel tree updates in tests

zebra-chain/src/block/arbitrary.rs

@@ -449,6 +449,9 @@ impl Block {
                         // would be awkward since the genesis block is handled separatedly there.
                         // This forces us to skip the genesis block here too in order to able to use
                         // this to test the finalized state.
+                        //
+                        // TODO: run note commitment tree updates in parallel rayon threads,
+                        //       using `NoteCommitmentTrees::update_trees_parallel()`
                         if generate_valid_commitments && *height != Height(0) {
                             for sapling_note_commitment in transaction.sapling_note_commitments() {
                                 sapling_tree.append(*sapling_note_commitment).unwrap();