Ergonomics of calling get_test_data from fv3core and fv3gfs-physics directories

[jdahm]

As a consumer I find it odd that we call get_test_data for the driver from the fv3core directory with TARGET=driver. Should this be moved to the driver directory at some point?

[mcgibbon]

From the physics directory is normally where you'd download with TARGET=driver, but it's duplicated in between there and dycore.

We should avoid making a third copy of this infrastructure.

Once we remove the duplication of work in these two places (already have for the test logic in #209, we may also want to do this for the docker images), it should be easier to consolidate all of these into one test entrypoint that lets you select e.g. "I want to run the dycore translate tests only" but is also able to run all tests.