To get an idea of how we're doing, we need to track test code coverage long term. Jenkins only stores history for about 30 days.
See https://tools.wmflabs.org/coverage/mediawiki/chart.png as a proof of concept, which is generated via clover.xml files that aren't ever deleted. It currently has all of the history that jenkins had ~3 days ago and grabs each day's new coverage run.
Source code is currently at https://gitlab.com/legoktm/tool-coverage (will be moved to Phab once striker is fixed)