remove reliance on remote datasets in tests

[jameslamb]

Description

One test here downloads a file from a non-RAPIDS repo (https://github.com/loganpowell/census-geojson) on every run:

https://github.com/rapidsai/cuxfilter/blob/6237ba143c1097498018fef82c4c3a6bd6250a8d/python/cuxfilter/tests/charts/deckgl/test_deckgl.py#L20-L34

That should be factored out.

Benefits of this work

• improves CI stability (ref: https://github.com/rapidsai/cuxfilter/pull/649#pullrequestreview-2579507810)
• reduces the risk of disruption for developers running tests locally (especially in environments with weak internet connection)

Acceptance Criteria

• tests do not depend on downloading external data (where avoidable)

Approach

Some possible alternatives:

• using synthetic geojson data generated at test time
• using test data checked into source control here (ideally a smaller file, if appropriate... that file is 2.1MB)
• removing the test (if on a re-review, it seems to not be useful)

Notes

N/A