add roundtrip tests which vary the number of partitions

See tests [here](https://github.com/datafusion-contrib/datafusion-distributed/blob/0ec389d094405c4133ec78afcc3ab9e76005b22d/src/codec.rs#L313-L323) and related comment https://github.com/datafusion-contrib/datafusion-distributed/pull/57#discussion_r2236589797