tokio-test: add tokio_test::io mock builder

tokio-test/tests/io.rs

@@ -0,0 +1,26 @@
+#![deny(warnings, rust_2018_idioms)]
+#![feature(async_await)]
+
+use tokio::io::{AsyncReadExt, AsyncWriteExt};
+use tokio_test::io::Builder;
+
+#[tokio::test]
+async fn read() {
+    let mut mock = Builder::new().read(b"hello ").read(b"world!").build();
+
+    let mut buf = [0; 256];
+
+    let n = mock.read(&mut buf).await.expect("read 1");
+    assert_eq!(&buf[..n], b"hello ");
+
+    let n = mock.read(&mut buf).await.expect("read 2");
+    assert_eq!(&buf[..n], b"world!");
+}
+
+#[tokio::test]
+async fn write() {
+    let mut mock = Builder::new().write(b"hello ").write(b"world!").build();
+
+    mock.write_all(b"hello ").await.expect("write 1");
+    mock.write_all(b"world!").await.expect("write 2");
+}