Unit tests are very very slow

[brews]

Unit tests run in CI have become very very slow. The can take about 15 minutes to complete.

I suspect dodola.services.validate() using dask.delayed is the culprit. Dask was used here as a hack to work around memory problems (see #126). The drawback to this hack has been that simple unit tests on Datasets covering a long time range take much more time to run.

If this is the case I see two options:

1. Re-write validation to work around the original memory problems in dodola validate-dataset reads in entire zarr store? #126 while also making unit tests complete faster. i.e. replace the hack solution with a "real" solution.
2. Find and refactor the one or two long-running unit tests so that they test the same behavior, but run faster.