Use 2018 edition for cargotest

src/tools/cargotest/Cargo.toml

@@ -2,6 +2,7 @@
 name = "cargotest2"
 version = "0.1.0"
 authors = ["Brian Anderson <banderson@mozilla.com>"]
+edition = "2018"
 
 [[bin]]
 name = "cargotest"

src/tools/cargotest/main.rs

@@ -1,3 +1,5 @@
+#![deny(rust_2018_idioms)]
+
 use std::env;
 use std::process::Command;
 use std::path::{Path, PathBuf};