Summary: This adds some pretty nice unit test coverage analysis for our codebase. Especially useful for external pull requests as a gentle nudge to make sure they don't regress the topline number. To learn more, check out https://github.com/coverallsapp/github-action Pull Request resolved: https://github.com/facebook/flipper/pull/1392 Test Plan: {F245038134} {F245038132} https://coveralls.io/builds/32250084 Reviewed By: jknoxville Differential Revision: D22664361 Pulled By: passy fbshipit-source-id: 4dc1fc79677da41a9d3ea198224c5678ae156ad4