test(client): Re-enable the tests/client.rs test

These tests were temporarily disabled during the migration to the
`std::future::Future` type that's part of the stable Rust now.

This commit updates the tests after the breaking changes and makes them
pass again.

src/body/body.rs

@@ -553,15 +553,16 @@ mod tests {
     // FIXME: re-implement tests with `async/await`, this import should
     // trigger a warning to remind us
     use crate::Error;
+    use futures_util::try_stream::TryStreamExt;
+    use tokio::runtime::current_thread::Runtime;
 
-    /*
     use super::*;
     #[test]
     fn test_body_stream_concat() {
         let body = Body::from("hello world");
 
-        let total = body.concat2().wait().unwrap();
+        let mut rt = Runtime::new().unwrap();
+        let total = rt.block_on(body.try_concat()).unwrap();
         assert_eq!(total.as_ref(), b"hello world");
     }
-    */
 }