add unit test suite to check fct_* behaviour when input is not a factor

[ayogasekaram]

closes #18

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Pre-review Checklist (if item does not apply, mark is as complete)

• All GitHub Action workflows pass with a ✅
• PR branch has pulled the most recent updates from master branch: usethis::pr_merge_main()
• If a bug was fixed, a unit test was added.
• If a standalone script was updated, a comment is added to the script header (changelog) AND the last-updated field has been updated.
• Code coverage is suitable for any new functions/features (generally, 100% coverage for new code): devtools::test_coverage()
• Request a reviewer

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

• If a bug was fixed, a unit test was added.
• If a standalone script was updated, a comment is added to the script header (changelog) AND the last-updated field has been updated.
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• Code coverage is suitable for any new functions/features: devtools::test_coverage()

When the branch is ready to be merged:

• All GitHub Action workflows pass with a ✅
• Approve Pull Request
• Merge the PR. Please use "Squash and merge" or "Rebase and merge".
• Create an issue in any repositories using {standalone} to update the standalone scripts.

[ddsjoberg]

Thank you so much @ayogasekaram !!