fix: default regex & write basic tests

[kelly-sovacool]

Changes

• Warn if multiple input counts or sample files are found.
• Make sure the regex accepts an optional .gz.
• Rename results output to moo-raw.rds.
• Write basic tests.

Issues

fixes #4
fixes #5

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Run the capsule in Code Ocean and ensure it completes successfully with no errors.
• Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Follow the style described in https://style.tidyverse.org/news.html
• The test workflow succeeds on the latest commit.