Tests for order by with nested fields (Fixes #53)

implementation not changed, tests did not turn up any problems
Not sure what you mean. The idea is that `exp` in each case is a query that we know is compiled correctly, and should mean the same thing as the test query. Do you think that's a bad approach, or do you not think that these `exp` queries are well chosen?

It's tedious to write these expected logical plans, and they'll require maintenance whenever we change our approach, so I was hoping to avoid some of that here since there should be a clearly equivalent, unambiguous query for each case, right?