message: Minor test fix (error text comparison)
  Serde's errors for missing fields are a lil' different than
  the ones from our Deserializer.
diff: @@ -687,10 +687,7 @@ Caused by:
  f3: i64,
  big: i64,
  }
  - assert_error(
  - config.get::<S>("S").unwrap_err(),
  - "missing config key `S.f3`",
  - );
  + assert_error(config.get::<S>("S").unwrap_err(), "missing field `f3`");
  }
  #[cargo_test]
  