Split test deps & move JuliaFormatter to action

[gdalle]

Supersedes #57

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.16%. Comparing base (a1fb161) to head (fcf4d51).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   94.90%   95.16%   +0.26%     
==========================================
  Files           9        9              
  Lines         471      476       +5     
==========================================
+ Hits          447      453       +6     
+ Misses         24       23       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.