Paired test ordering

[ddsjoberg]

What changes are proposed in this pull request?

• Fix in add_difference() for pair t-tests. Previously, the sign of the reported difference depended on which group appeared first in the source data. Function has been updated to consistently report the difference as the first group mean minus the second group mean. (Paired t-tests do not respect group levels #1557)

If there is an GitHub issue associated with this pull request, please provide link.
closes #1557

---

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

• Ensure all package dependencies are installed by running renv::install()
• 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.
• If an update was made to tbl_summary(), was the same change implemented for tbl_svysummary()?
• If a new function was added, function included in _pkgdown.yml
• If a bug was fixed, a unit test was added for the bug check
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• 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.
• R CMD Check runs without errors, warnings, and notes
• usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged into master:

• 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).
• Increment the version number using usethis::use_version(which = "dev")
• Run codemetar::write_codemeta()
• Run usethis::use_spell_check() again
• Approve Pull Request
• Merge the PR. Please use "Squash and merge".