v2.0 Add unit tests for `as_flex_table`

What changes are proposed in this pull request?

Add v2.0 unit tests for as_flex_table.

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".

ddsjoberg / gtsummary