Test run not failing with broken tests

[lholmquist]

Describe the Bug
While creating the PR for #301, i added a test that should fail the build. It does fail the test, but the test suite still thinks it passes. The error shows up as an unhandled promise rejection: https://github.com/cloudevents/sdk-javascript/pull/302/checks?check_run_id=951373768#step:6:330

Expected Behavior
The expected behavior is that CI should fail

Additional context
This isn't a problem with the library source code. It only happens in tests that are using promises. It could be someting with mocha and how it handles promise rejections. But that is my guess

edit: After doing a little testing, if we just add a return inside the tests, then things break/pass like a normal tets