Reorganize files

[olivroy]

To have a standardized way to test for output and facilitate workflows.

This is the first step and doesn't seem to conflict with open PRs.

I plan to do more in the future, but this is a first step.

The idea is to improve workflow with usethis::use_r() / use_test()

• Standardized latex tests to the test-l_* prefix
• Standardized rtf tests to the test-rtf_* prefix (noticed 2 similar test files that I combined
• Move fmt() into own file along with all helper functions for formatting (previously found in utils_formatters.R

[rich-iannone]

Thank you!