Add AST pretty-printer tests involving attr on binary operation

This test currently fails (as expected).

    --- stderr -------------------------------
    Pretty-printer lost necessary parentheses
      BEFORE: #[attr] (1 + 1)
       AFTER: #[attr] 1 + 1

    Pretty-printer lost necessary parentheses
      BEFORE: #[attr] (1 as T)
       AFTER: #[attr] 1 as T

    Pretty-printer lost necessary parentheses
      BEFORE: #[attr] (x = 1)
       AFTER: #[attr] x = 1

    Pretty-printer lost necessary parentheses
      BEFORE: #[attr] (x += 1)
       AFTER: #[attr] x += 1
    ------------------------------------------

Author: dtolnay

tests/ui-fulldeps/pprust-parenthesis-insertion.rs

@@ -92,6 +92,12 @@ static EXPRS: &[&str] = &[
     "#[attr] loop {}.field",
     "(#[attr] loop {}).field",
     "loop { #![attr] }.field",
+    // Attributes on a Binary, Cast, Assign, and AssignOp expression require
+    // parentheses.
+    "#[attr] (1 + 1)",
+    "#[attr] (1 as T)",
+    "#[attr] (x = 1)",
+    "#[attr] (x += 1)",
     // Grammar restriction: break value starting with a labeled loop is not
     // allowed, except if the break is also labeled.
     "break 'outer 'inner: loop {} + 2",