Add testing support for gunicorn. While documentation has been provided for using ddtrace with gunicorn, there were no tests ensuring that the library actually works with it.

Reviewer Checklist

• Title is accurate.
• Description motivates each change.
• No unnecessary changes were introduced in this PR.
• Avoid breaking API changes unless absolutely necessary.
• Tests provided or description of manual testing performed is included in the code or PR.
• Release note has been added and follows the library release note guidelines, or else changelog/no-changelog label added.
• All relevant GitHub issues are correctly linked.
• Backports are identified and tagged with Mergifyio.