e9eb5e6a

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Add coverage threshold and requireCoverage checks

Add assertCoverageThreshold and requireCoverage as standalone nix
functions, and wire them into makeCoverageReport as optional arguments
(threshold and mustCover). mustCover defaults to true so that builds
fail when no expressions are covered at all, catching accidentally
disabled tests.