Add a testcase for join where one stream remains pending forever

This currently hangs (danieldg#12).

src/join.rs

@@ -382,6 +382,11 @@ where
 
 #[cfg(test)]
 mod test {
+    extern crate alloc;
+
+    use alloc::boxed::Box;
+    use futures_util::stream::pending;
+
     use crate::join;
     use crate::FromStream;
     use crate::OrderedStreamExt;
@@ -414,4 +419,18 @@ mod test {
             }
         });
     }
+
+    #[test]
+    fn join_1_pending() {
+        futures_executor::block_on(async {
+            let stream1 = FromStream::with_ordering(pending::<Message>(), |m| m.serial);
+            let stream2 = FromStream::with_ordering(
+                futures_util::stream::once(Box::pin(async { Message { serial: 1 } })),
+                |m| m.serial,
+            );
+            let mut joined = join(stream1, stream2);
+            let msg = joined.next().await.unwrap();
+            assert_eq!(msg.serial, 1);
+        });
+    }
 }