test: add a block_on function to tokio-test (#1431)

tokio-test/tests/block_on.rs

@@ -0,0 +1,28 @@
+#![warn(rust_2018_idioms)]
+#![feature(async_await)]
+
+use std::time::{Duration, Instant};
+use tokio_test::block_on;
+use tokio_timer::Delay;
+
+#[test]
+fn async_block() {
+    assert_eq!(4, block_on(async { 4 }));
+}
+
+async fn five() -> u8 {
+    5
+}
+
+#[test]
+fn async_fn() {
+    assert_eq!(5, block_on(five()));
+}
+
+#[test]
+fn delay() {
+    let deadline = Instant::now() + Duration::from_millis(100);
+    let delay = Delay::new(deadline);
+
+    assert_eq!((), block_on(delay));
+}