message: Remove std::fs::read_to_string reimplementation from tests
diff: use serde::Deserialize;
  use serde_json as json;
  -use std::fs::File;
  +use std::fs::{read_to_string, File};
  use std::io::{self, Read};
  use std::path::Path;
  @@ -75,16 +75,6 @@ where
  res
  }
  -fn read_string<P>(path: P) -> Result<String, ::std::io::Error>
  -where
  - P: AsRef<Path>,
  -{
  - let mut res = String::new();
  - File::open(path.as_ref()).and_then(|mut f| f.read_to_string(&mut res))?;
  -
  - Ok(res)
  -}
  -
  #[derive(Deserialize, Default)]
  struct RefConfig {
  history_size: u32,
  @@ -97,9 +87,9 @@ impl EventListener for Mock {
  fn ref_test(dir: &Path) {
  let recording = read_u8(dir.join("alacritty.recording"));
  - let serialized_size = read_string(dir.join("size.json")).unwrap();
  - let serialized_grid = read_string(dir.join("grid.json")).unwrap();
  - let serialized_cfg = read_string(dir.join("config.json")).unwrap();
  + let serialized_size = read_to_string(dir.join("size.json")).unwrap();
  + let serialized_grid = read_to_string(dir.join("grid.json")).unwrap();
  + let serialized_cfg = read_to_string(dir.join("config.json")).unwrap();
  let size: SizeInfo = json::from_str(&serialized_size).unwrap();
  let grid: Grid<Cell> = json::from_str(&serialized_grid).unwrap();
  