A sample package in R
Install devtools and testthat:
install.packages('devtools')
install.packages('testthat')
Load devtools library:
library(devtools)
Create basic testing infrastructure:
# navigate to your project dir
setwd('path/to/project')
test('.') # will ask you to create testing infrastructure, press 1 Enter
Create tests in files named test*.R inside tests/testthat,
and run again test('.').
See the docs of testthat on how to write unit tests.