test_join_big_small materializes the small dataframe before merging, which circumvents a distributed join.

https://github.com/coiled/coiled-runtime/blob/ef1fe4e983afc29ba80b8adbb056cefc27788e04/tests/benchmarks/test_join.py#L56-L58

While this is a reasonable thing for a user to do if they know the size of their data and want to optimize their code, we should also test the performance without an early materialization.