Re-enable upload of coverage files to codecov

[richard-cox]

• in the test workflow we use the action codecov/codecov-action@v3 to upload unit test coverage to codecov servers
• the action has been approved for use, but with caveats (it must be in it's on workflow
• given timeline the action will be removed from test but the functionality needs to come back
• we can either
  • create a new workflow triggered on test workflow, download artefacts (these are already uploaded) and then upload to codecov
  • OR
  • use the codecov cli to upload from within test