API mocking for package tests

[nickdickinson]

API mocking would be useful for the following:

• Reduce the number of API calls during testing of non-mutating interactions and enable periodic update of mocking as needed
• Run vignettes using mocking to avoid authentication and the need to keep resources online making them easier to maintain as the R package changes
• Enabling the tests to run in CRAN so that the package can be submitted to CRAN. Alternatively we could disable testing in CRAN for tests that use the API/internet.
• Potentially use also in the Roxygen examples

The space has grown considerably with the following useful packages:

• vcr (httr and httr2 supported
• httptest (for httr) or httptest2 (for httr2)
• webmockr

This is a good introduction to API mocking in R: https://books.ropensci.org/http-testing/index.html