Testing coverage

[minghinmatthewlam]

Context and scope
Right now we have unit tests as part of CI. We want to update CI to print out a test coverage report, and try to add tests to reach a standard of test coverage we're happy with, likely measured through a percentage.

Discussion and alternatives
subnet-evm ci with coverage report: https://github.com/ava-labs/subnet-evm/blob/master/.github/workflows/ci.yml, coverage script https://github.com/ava-labs/subnet-evm/blob/master/scripts/coverage.sh

Open questions
What is the standard for our test coverage?
Do we want to require in CI to be above a threshold to pass CI?