To cite package ‘testthat’ in publications use:

  Wickham H (2011). “testthat: Get Started with Testing.” _The R
  Journal_, *3*, 5-10.
  <https://journal.r-project.org/archive/2011-1/RJournal_2011-1_Wickham.pdf>.

A BibTeX entry for LaTeX users is

  @Article{,
    author = {Hadley Wickham},
    title = {testthat: Get Started with Testing},
    journal = {The R Journal},
    year = {2011},
    volume = {3},
    pages = {5--10},
    url = {https://journal.r-project.org/archive/2011-1/RJournal_2011-1_Wickham.pdf},
  }