Improving speed of CI testing

[chrisiacovella]

Pull Request Summary

This PR is focused on trying different strategies to speed up our CI testing.

One issue is that the training tests are very memory intensive and only a single can be run at a time. I've tried putting these into a single xdist group (which will mean they will all execute on the same worker thus multiple will not be running at the same time); I've upped the number of workers to 4, since many of the other tests are not memory intensive (or cpu intensive), and can more can easily run concurrently.

this drops CI testing from 43 minutes to 23 minutes.

Associated Issue(s)

• Speed up CI #379

Pull Request Checklist

• Issue(s) raised/addressed and linked
• Includes appropriate unit test(s)
• Appropriate docstring(s) added/updated
• Appropriate .rst doc file(s) added/updated
• PR is ready for review

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.85%. Comparing base (4f7c718) to head (ca03092).

🚀 New features to boost your workflow:

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