The tests themselves are stored in package-specific directories in tests.
Some datasets that are necessary for the tests are stored in datasets, together with any code necessary for generating them.
Datasets and the generating code should be stored with the same name, with different extensions, i.e. demo.R for the code and demo.csv for the dataset itself.
Data may also be generated in the test-function.R or setup-package.R files if they are not of general interest for testing multiple packages.