Add tests for density estimators

[ablaom]

In MLJBase 1.11, support was added for evaluating the performance of density estimators. This PR adds tests for density estimators wrapped using TunedModel.

This PR does not touch /src code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.34%. Comparing base (2a6208b) to head (37770a6).
⚠️ Report is 9 commits behind head on dev.

❌ Your project status has failed because you have indirect coverage changes. Learn more about Unexpected Coverage Changes and reasons for indirect coverage changes.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #228      +/-   ##
==========================================
- Coverage   87.85%   87.34%   -0.51%     
==========================================
  Files          13       13              
  Lines         667      664       -3     
==========================================
- Hits          586      580       -6     
- Misses         81       84       +3     

☔ 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.