Unittest runner does not parallelize

[bigjools]

My test suite has over 3500 tests, which take ~400 seconds to run in vscode. If I run using stestr to make use of my 12 cores, they complete in less than 30 seconds.

Can you please add a change to parallelize by default based on the number of cores available.

[github-actions]

Thanks for the feature request! We are going to give the community 60 days from when this issue was created to provide 7 👍 upvotes on the opening comment to gauge general interest in this idea. If there's enough upvotes then we will consider this feature request in our future planning. If there's unfortunately not enough upvotes then we will close this issue.