Include more unit tests for existing check functions

[sarabodach]

From @harriscw -

For those interested in contributing to unit tests here's how to get started:

• clone the repo
• run renv::restore to install the same set of packages being used
• create a branch from devel
• # to create some unit tests here
• Take examples from the roxygen header and turn them into unit tests in the tests/testtthat folder. There are already some reference unit tests in there for guidance
• PR your work

[sarabodach]

https://testthat.r-lib.org/index.html
https://r-pkgs.org/testing-basics.html

This edition applies to sdtmchecks:
https://testthat.r-lib.org/articles/third-edition.html

Guidance from Admiral package:
https://pharmaverse.github.io/admiraldev/devel/articles/unit_test_guidance.html#set-up-the-test-script

[sarabodach]

Thanks @laywang142 , @J-Lox , @harriscw !