* chore: update how we track coverage during unit tests
The old method of tracking coverage was causing issues. If a test failed
on CI, that test's failure would be lost because the test failing would
in turn cause the coverage to fail, but the `process.exit(1)` in the
coverage code caused Mocha to not output anything useful.
Instead the coverage checker now:
* tracks the coverage in memory in a Map (this hasn't changed)
* after all tests, writes that to disk in test/coverage.json (which is
gitignored)
* we then run a single Mocha test that asserts every method was called.
This means if the test run fails, the build will fail and give the error
about that test run, and that output won't be lost when the coverage
then fails too.
Co-authored-by: Mathias Bynens <mathias@qiwi.be>
Rather than maintain our own test runner we should instead lean on the community and use Mocha which is very popular and also our test runner of choice in DevTools too.
Note that this commit doesn't remove the TestRunner source as it's still used for other unit tests, but they will be updated in a future PR and then we can remove the TestRunner.
The main bulk of this PR is updating the tests as the old TestRunner passed in contextual data via the `it` function callback whereas Mocha does not, so we introduce some helpers for the tests to make it easier.
