chore: update how we track coverage during unit tests (#5779)

* 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>

.gitignore

@@ -15,3 +15,4 @@ yarn.lock
 /utils/browser/puppeteer-web.js
 /index.d.ts
 /lib
+test/coverage.json