fuzz: update taptree regression test to also check control blocks

This is a bit of a pain because the old lookup-by-script behavior
doesn't map cleanly onto the new iterate-through-everything behavior.
But this test is definitely useful (the unit tests in the previous
commit came from it.)

Author: apoelstra

fuzz/fuzz_targets/regression_taptree.rs

@@ -1,3 +1,5 @@
+use std::collections::{HashMap, HashSet};
+
 use descriptor_fuzz::FuzzPk;
 use honggfuzz::fuzz;
 use miniscript::descriptor::Tr;
@@ -27,6 +29,22 @@ fn do_test(data: &[u8]) {
                 new_si.output_key(),
                 "merkle root mismatch (left is old, new is right)",
             );
+
+            // Map every leaf script to a set of all the control blocks
+            let mut new_cbs = HashMap::new();
+            for leaf in new_si.leaves() {
+                new_cbs
+                    .entry(leaf.script())
+                    .or_insert(HashSet::new())
+                    .insert(leaf.control_block().clone());
+            }
+            // ...the old code will only ever yield one of them and it's not easy to predict which one
+            for leaf in new_si.leaves() {
+                let old_cb = old_si
+                    .control_block(&(leaf.script().into(), leaf.leaf_version()))
+                    .unwrap();
+                assert!(new_cbs[leaf.script()].contains(&old_cb));
+            }
         }
     }
 }