Speed up bulk download tests

[chriscarrollsmith]

I think there are more places we should be using mocks with pre-downloaded sample data rather than actual API calls in the bulk test functions.

[chriscarrollsmith]

I also encountered a couple 503 (service temporarily unavailable) errors while running the test suite, which would be very annoying if it happened during a cloud CRAN submission test. Another good reason to implement as much mocking as possible. This looks very promising for that purpose:

https://cran.r-project.org/web/packages/httptest2/vignettes/httptest2.html
https://cran.r-project.org/web/packages/httptest2/vignettes/redacting.html

EDIT: I experimented with httptest2 for this, and unfortunately it was pretty awful because it saves, stores, and loads the blob files using the least efficient imaginable format. For most, if not all, of our tests, I think it will be better to mock download_file manually.