Use BTreeMap in test code

Instead of `HashMap` we can use a `BTreeMap` in unit test code, the
functionality is exactly the same although `BTreeMap` does not have a
`with_capacity` constructor.

Author: tcharding

src/descriptor/mod.rs

@@ -1501,7 +1501,7 @@ mod tests {
             witness: Witness::default(),
         };
         let satisfier = {
-            let mut satisfier = HashMap::with_capacity(2);
+            let mut satisfier = BTreeMap::new();
 
             satisfier.insert(
                 a,