chore: change default lint level to warning and deny warnings in CI (#1416)

tokio-test/tests/clock.rs

@@ -1,5 +1,5 @@
 #![cfg(feature = "broken")]
-#![deny(warnings, rust_2018_idioms)]
+#![warn(rust_2018_idioms)]
 
 use futures::Future;
 use std::time::{Duration, Instant};

tokio-test/tests/io.rs

@@ -1,4 +1,4 @@
-#![deny(warnings, rust_2018_idioms)]
+#![warn(rust_2018_idioms)]
 #![feature(async_await)]
 
 use tokio::io::{AsyncReadExt, AsyncWriteExt};

tokio-test/tests/macros.rs

@@ -1,5 +1,5 @@
 #![cfg(feature = "broken")]
-#![deny(warnings, rust_2018_idioms)]
+#![warn(rust_2018_idioms)]
 
 use futures::{future, Async, Future, Poll};
 use tokio_macros::{assert_not_ready, assert_ready, assert_ready_eq};