Possible way to add coverage to run-all-tests

@mwall I've added pytest-cov to search-api (pull request https://github.com/LandRegistry/search-api/pull/10). Not knowing the outcome of all the pull request shenanigans, I'm thinking this would be a reasonable idea. 

I've actually crashed in the change on the-feeder master as well.

One thing that is not connected yet is the report side of things in coveralls.io. I've logged in over there but it's only showing my individual repos not the org ones that I belong to. https://github.com/lemurheavy/coveralls-public/issues/199