Can I just check what you mean by this though? We already run unit tests as part of CI, and require 100% pass rate:

... where the action 'perform unit tests' will fail if 100% of the tests do not pass:

As we add in more tests these will be automatically added to the list.

But do you mean something else with this issue?

All tests definitely need to pass, but this is about how much code is tested.

Test-coverage checkers tell us how much of the total codebase (by lines) is covered by at least one test. We can set a benchmark, like 50%, and not allow merging of branches that increase the amount of untested code to 50%.

Ah, gotcha 👍

Here's an example of a coverage action: https://github.com/marketplace/actions/python-coverage

Once the repo is open (#265), there are a bunch of free tools we could use, e.g. coveralls.io.