chore: include tests in prettier formatting

[freema]

Summary

• Extend format and format:check npm scripts to include tests/**/*.ts
• Apply prettier to all existing test files (21 files touched, style-only changes)

Follow-up to the discussion in #59.

Test plan

• npx prettier --check "src/**/*.ts" "tests/**/*.ts" passes
• No logic changes, only whitespace/formatting

[juliandescottes]

Thanks for changing this. It's better to do it as early as possible. IMO it doesn't really matter whether we deal with test or "source" files. As long as they are files which can be linted/prettified and are going to be updated by more than 1 person, it's going to be useful in the long run.