Add code coverage to ckanext-dcat_usmetadata extension

[starsinmypockets]

As a code maintainer I want to monitor code coverage so that I can maintain a high standard of testing for the extension

Accepance Criteria

• [ ] Code coverage report can be generated locally
• [ ] CI fails on reduced code coverage for commit

Tasks

• [ ] Get code coverage and report working locally
• [ ] Get code coverage to work with circleci

[starsinmypockets]

From Slack:

[starsinmypockets]

cc @woodt we can continue discussion here?

I also think adding coverage report with no failure case is fine for now.