add coverage report artifact and minimum coverage

[shapiromatron]

Add a minimum coverage of 91.3%. This will fail in CI/CD if we go below this number. It's expected we'll revisit this number and hopefully increase it quarterly.

If you provide a --fail-under value, the total percentage covered will be compared to that value. If it is less, the command will exit with a status code of 2, indicating that the total coverage was less than your target. This can be used as part of a pass/fail condition, for example in a continuous integration server. This option isn’t available for annotate.

https://coverage.readthedocs.io/en/latest/cmd.html#html-reporting-coverage-html