[SPARKR-143] Sort lists before comparing in unit tests

Since Spark doesn't guarantee that shuffle results will always be in the same order, we need to sort the results before comparing for deterministic behavior. 

Couple of points still TODO
1. I need to take a look to make sure there are no other usages that I missed out.
2. For some tests it might be good to also sort the values per key. 

/cc @sun-rui