Implement `ghc-testsuite` report diff script and run on CI

Currently, to spot regressions in one of the ghc-testsuite jobs, we need to manually download the CSV reports from CircleCI and perform diffing based on some ad hoc script, and since it's not mechanized we don't perform this for every PR, increasing the chance of regression. We should implement:

• Script to diff two CSV reports, locating the fixes & regressions, taking into account the possibility of ghc-testsuite tests may get added or deleted
• Script to query CircleCI artifacts and download available reports from parent commits
• Run the scripts on CI. We should probably fail the job if there's regression, provided that we got rid of non-determinism (like in #284) in test runs