Add missing lines to coverage report

Description of feature request

pytest-coverage can output a column listing the specific lines which were not covered. We're currently (implicitly) using the default report, --cov-report term, but we simply have to swap this for --cov-report term-missing to add the extra info.

https://pytest-cov.readthedocs.io/en/latest/reporting.html

Use cases

This helps target testing efforts on lines that have not been covered.

Benefits

Better targeting of tests.

Drawbacks

The CI/CD job logs will be a bit wider. If this is a problem we could instead keep the current simplified terminal output, and add --cov-report xml or --cov-report html to add the more detailed info in a job artifact.

Suggested solutions

Add --cov-report term-missing to pytest's arguments in .gitlab-ci.yml. I'll have a merge request momentarily.