fetch test data from mirror

[cyrush]

Temporary improvement for CI checks.

[cyrush]

This is currently running tests using test data from an http mirror, w/o baselines (which of course will fail).

The data doesn't change often (with the common case being adding new datasets), where the baselines will change, so we need a separate strategy for baselines.

We could disable the hard requirement of the test suite passing, and rely on manual human inspection of the results as part of the PR process for now.