message: Learn to use Project::read_file & String::insert
diff: -use std::fs::{ File, self };
  +use std::fs::File;
  use std::io::prelude::*;
  use support::registry::Package;
  @@ -381,19 +381,15 @@ fn preserve_top_comment() {
  p.cargo("update").run();
  - let lockfile_path = p.root().join("Cargo.lock");
  -
  - let mut lockfile = String::new();
  - let mut f = File::open(&lockfile_path).unwrap();
  - lockfile.push_str("# @generated\n");
  - f.read_to_string(&mut lockfile).unwrap();
  + let mut lockfile = p.read_file("Cargo.lock");
  + lockfile.insert_str(0, "# @generated\n");
  println!("saving Cargo.lock contents:\n{}", lockfile);
  p.change_file("Cargo.lock", &lockfile);
  p.cargo("update").run();
  - let lockfile2 = fs::read_to_string(&lockfile_path).unwrap();
  + let lockfile2 = p.read_file("Cargo.lock");
  println!("loaded Cargo.lock contents:\n{}", lockfile2);
  let first_line = lockfile2.lines().into_iter().next().unwrap();
  