v2.0 add unit-tests for tbl_survfit for add_n, add_nevent, add_p

[zdz2101]

What changes are proposed in this pull request?

If there is an GitHub issue associated with this pull request, please provide link.

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Reviewer Checklist (if item does not apply, mark is as complete)

• [x] Ensure all package dependencies are installed by running renv::install()
• [x] PR branch has pulled the most recent updates from master branch. Ensure the pull request branch and your local version match and both have the latest updates from the master branch.
• [x] If an update was made to tbl_summary(), was the same change implemented for tbl_svysummary()?
• [x] If a new function was added, function included in _pkgdown.yml
• [x] If a bug was fixed, a unit test was added for the bug check
• [x] Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• [x] Code coverage is suitable for any new functions/features. Review coverage with withr::with_envvar(new = c("NOT_CRAN" = "true"), covr::report()). Begin in a fresh R session without any packages loaded.
• [x] R CMD Check runs without errors, warnings, and notes
• [x] usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged into master:

• [x] Update NEWS.md with the changes from this pull request under the heading "# gtsummary (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end update (see NEWS.md for examples).
• [x] Increment the version number using usethis::use_version(which = "dev")
• [x] Run codemetar::write_codemeta()
• [x] Run usethis::use_spell_check() again
• [ ] Approve Pull Request
• [ ] Merge the PR. Please use "Squash and merge".

[ddsjoberg]

@zdz2101 is this ready for review?

[zdz2101]

@zdz2101 is this ready for review?

Ahh yes I forgot to assign you, this is ready!

[ddsjoberg]

Hey! I am going to hold off reviewing these updates until we get the tests for tbl_survfit() merged. If it happens we have to make changes to the primary function, that may cause downstream issues in these fns.

[zdz2101]

Closing in favor of of #1838