Speed up testing - Githubissues

Prework

[x] Read and agree to the code of conduct and contributing guidelines.
[x] If there is already a relevant issue, whether open or closed, comment on the existing thread instead of posting a new issue.
[x] For any problems you identify, post a minimal reproducible example like this one so the maintainer can troubleshoot. A reproducible example is:
- [x] Runnable: post enough R code and data so any onlooker can create the error on their own computer.
- [x] Minimal: reduce runtime wherever possible and remove complicated details that are irrelevant to the issue at hand.
- [x] Readable: format your code according to the tidyverse style guide.

Before any of the main tests, compile the example model once and stash it in tempdir().
Copy the compiled model to the testing directory.
Add an environment variable to skip tests with compile = "copy".