Ignore unnecessary_box_returns pedantic clippy lint in test

    warning: boxed return of the sized type `rustc_ast::Expr`
       --> tests/test_precedence.rs:203:64
        |
    203 | fn librustc_parenthesize(mut librustc_expr: Box<ast::Expr>) -> Box<ast::Expr> {
        |                                                                ^^^^^^^^^^^^^^ help: try: `rustc_ast::Expr`
        |
        = help: changing this also requires a change to the return expressions in this function
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_box_returns
        = note: `-W clippy::unnecessary-box-returns` implied by `-W clippy::pedantic`
        = help: to override `-W clippy::pedantic` add `#[allow(clippy::unnecessary_box_returns)]`

Author: dtolnay

tests/test_precedence.rs

@@ -28,7 +28,8 @@
     clippy::match_wildcard_for_single_variants,
     clippy::needless_lifetimes,
     clippy::too_many_lines,
-    clippy::uninlined_format_args
+    clippy::uninlined_format_args,
+    clippy::unnecessary_box_returns
 )]
 
 extern crate rustc_ast;