Finalise how covtracer is represented in v1.0 of the gh-action

From issue #8

The `covtracer` output here is pretty generic. We can do some filtering if we want to present other information. By default, `NA`s are included for documented objects that are untested. As well, there may be situations where the `test_name` is unknown, but there is still testing code that prompts package execution (often this is test setup code, outside of any `test_that()` call).

In the `covtracer` README, I include these columns just to illustrate the missingness of code coverage. In a report, I would choose other, more meaningful columns - I would drop `test_name` and `count`, and instead filter out unexported documentation (`filter(is_exported = TRUE)`) to only show untested, exported functions as those are a better indicator of the user contract of a package. Just for the sake of clarity, it might be nice to have a clean "No untested behavior" if the resulting table has no records instead of just showing an empty table.

_Originally posted by @dgkf-roche in https://github.com/insightsengineering/thevalidatoR/issues/8#issuecomment-945941598_