message: Remove unnessecary `mut`.
diff: @@ -524,7 +524,7 @@ mod tests {
  #[test]
  fn headless_test_read_dir() {
  - let mut f = dummy_fs_for_tests();
  + let f = dummy_fs_for_tests();
  let dir_contents_size = f.read_dir("/").unwrap().count();
  assert!(dir_contents_size > 0);
  @@ -532,7 +532,7 @@ mod tests {
  #[test]
  fn headless_test_create_delete_file() {
  - let mut fs = dummy_fs_for_tests();
  + let fs = dummy_fs_for_tests();
  let test_file = path::Path::new("/testfile.txt");
  let bytes = b"test";
  @@ -552,7 +552,7 @@ mod tests {
  #[test]
  fn headless_test_file_not_found() {
  - let mut fs = dummy_fs_for_tests();
  + let fs = dummy_fs_for_tests();
  {
  let rel_file = "testfile.txt";
  match fs.open(rel_file) {
  @@ -575,7 +575,7 @@ mod tests {
  #[test]
  fn headless_test_write_config() {
  - let mut f = dummy_fs_for_tests();
  + let f = dummy_fs_for_tests();
  let conf = conf::Conf::new();
  // The config file should end up in
  // the resources directory with this
  