Track and fail coverage regressions in CI

[arlyon]

We have a coverage suite which isn't complete (yet), however we want to prevent regressions by tracking individual tests and ensuring each PR strictly increases the coverage.

To do this, we should split out the snapshots where possible to the smallest unit of granularity so we get insight at the tranformation level rather than at the file level. Then we need to store some metadata about which tests are expected to pass.