Simplify $lookup and $unionWith tests to avoid needing manual disposal

Currently a bunch of our unit tests for $lookup end with a line calling dispose, like this one.

This is a nuisance for readers and authors. It's easy to forget, and doesn't really add anything to the test. We should look into having a helper method return a $lookup stage via a unique_ptr with a custom destructor/deleter like Pipeline has or to use a ScopeGuard or something else to make this more automatic and less of a distraction.