I've been working on a tool, phpunit-patch-coverage, that tries to
provide test coverage reports for patches before they are merged.
Since it takes 2-3 hours to do a full coverage report for MediaWiki
core, the tool figures out what files were modified, and runs those tests.
For now I've set up a job that runs for MediaWiki core, and leaves a
comment like "PHP test coverage increased (or stayed the same)" or
"PHP test coverage decreased :-(". This job is slow so it is run
separately from unit tests to avoid delaying that feedback.
I'm sure there are bugs, please report them in the Phabricator
project, along with any feature requests on how we can make this
more useful. This should be considered beta quality software for now :-).