create small test and example datasets

[d-morrison]

Many of our examples and tests still take a long time to run, partially due to download times from OSF.
Let's:

• create minimal subset files that are small enough to include directly in the package
• swap them into tests and examples in place of downloads from OSF

(Let's keep using OSF data for the vignettes and articles, since those documents are meant to demonstrate actual research workflows.)