Add pretty printer test for use trees

Author: dtolnay

src/test/pretty/use-tree.rs

@@ -0,0 +1,17 @@
+// pp-exact
+// edition:2021
+
+#![allow(unused_imports)]
+
+use ::std::fmt::{self, Debug, Display, Write as _};
+
+use core::option::Option::*;
+
+use core::{cmp::{Eq, Ord, PartialEq, PartialOrd},
+    convert::{AsMut, AsRef, From, Into},
+    iter::{DoubleEndedIterator, ExactSizeIterator, Extend, FromIterator,
+    IntoIterator, Iterator},
+    marker::{Copy as Copy, Send as Send, Sized as Sized, Sync as Sync, Unpin
+    as U}, ops::{*, Drop, Fn, FnMut, FnOnce}};
+
+fn main() {}