Reinstate coverage to CI when testing is migrated to pytest

[CasperWA]

@sphuber Is it correctly understood that coverage/codecov has disappeared in the process of moving to GitHub Actions?

[sphuber]

Yes as mentioned in PR #3571 I omitted it on purpose. It was often buggy so we did not actually pay any attention to what it was saying. We put a big threshold on it just to prevent it from giving false positives. In addition, it was not even correctly registering all paths we were running. The move to pytest will allow us to run pytest-cov which should fix it. Maybe we look into it then again

[CasperWA]

Cool. I'll just put in a link to codecov-action here for the future implementation of this.
There is also a coveralls gh-action.

[ltalirz]

we might actually want to give it a try even before - if the information is usable, then it could be very helpful for improving test coverage for the "data/tools" commands during the coding week.

[CasperWA]

From short discussion at the coding week 2019 between @dev-zero, @giovannipizzi, and me:

• Move to codecov.
• Create GH Actions job to run on pushes to master and develop branches (on aiidateam/aiida-core) and for all PRs.
• Make sure to properly exclude test files and folders, and consider which other files/folders should be excluded.