handle {box} recommended method of testing private functions

[radbasa]

• Closes #98

Description

• Now can handle {box}-recommended method of testing private functions.
• Adds a first end-to-end test for this private function testing method.

Definition of Done

• [ ] The change is thoroughly documented.
• [ ] The CI passes (R CMD check, linter, unit tests, spelling).
• [ ] Any generated files have been updated (e.g. .Rd files with roxygen2::roxygenise())

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.37%. Comparing base (db2730f) to head (27d9317). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #99 +/- ## ======================================= Coverage 98.36% 98.37% ======================================= Files 21 21 Lines 797 799 +2 ======================================= + Hits 784 786 +2 Misses 13 13 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.