Ref: #304

Our code coverage is currently less than 45% 😬. That is not optimal, since this library has a lot of moving pieces, and many of which are untested.

I propose we:

• Enable code coverage checks (@nodejs/tsc needs to add Codecov's GitHub bot to this repository)
• Set patch coverage threshold to 80%
• Increase coverage to X%, set project coverage threshold to X% for every 10% until 90%.
  Currently: 40% Threshold / 50% Actual
• Disable patch coverage threshold, rely on the project threshold