dev: Add Codecov Test Analytics to pydantic-core

[ajay-sentry]

This PR aims to add Codecov's Test Analytics feature to the pydantic-core repository. With Test Analytics, there are 2 main features:

• First, there is a new PR notification showing failing, flaky, and slow tests.
• Second, there's a new "Tests" tab on the codecov UI which shows similar information in a dashboard view, but it can be searched and filtered on, and shows historical context as well.

More information can be found here: https://docs.codecov.com/docs/test-analytics#failed-test-reporting

For this PR the test tab for this branch is now populated: https://app.codecov.io/github/pydantic/pydantic-core/tests/ajay-sentry%3AAjay%2Fcodecov-test-analytics

As more PRs are merged the "main" branch view including the metric dashboard at the top will also be populated, similar to how Codecov's are: https://app.codecov.io/github/codecov/gazebo/tests/main

Change Summary

I saw we were already using the latest version of the codecov action, this PR adds the codecov test results action right below it, keeping the same styling in the file otherwise. I did also need to add the option of creating a junit xml to the pytest setup so that the test results action can pick it up and upload it to codecov though

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1684 will not alter performance

_{Comparing ajay-sentry:Ajay/codecov-test-analytics (b0aa790) with main (0a5bbfc)}

Summary

✅ 157 untouched benchmarks