How should we properly handle mocking of console.warn() in Jest tests?

[cn894]

This comes from a discussion in #70. We don't want the expected console.warn() calls to show up in Jest test runs. However, we don't want to swallow unexpected calls that could be legitimate problems.

This issue was created to act as a reminder to later explore this and re-examine the assumption that it will not pose problems in the short-term.