Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Purpose is twofold:

• Add mocking infrastructure which allows us to test R code given particular outputs of cmdstan without actually running cmdstan
• Prep for improved cpp option handling (Improve Handling of cpp_options #1022 )

I ran the CI on my fork (here). There are 3 failures, all of which I believe are unrelated to my change:

• macOS-latest (release) - issue in main, not sure root cause.
• windows-latest (release) - fix here: bump release rtools version #1076
• ubuntu-latest (release) - fix here: Workaround for https://github.com/r-lib/pak/issues/755 #1077

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Max Planck Institute of Animal Behavior

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)