Add some way to fail tests for unexpected warns & errors

[courajs]

Previously in the cardstack/cardstack test suite, we were throwing an error if we log.warn or log.error, when not inside an expectWarn/expectError block: https://github.com/cardstack/cardstack/blob/532f62a03e08917d77261552014f9eb4a5bef3f8/packages/test-support/prepare-node-tests.js

We were doing that by hooking into the print method of the logger, and throwing if we saw a warn or log come through.

We should either:

• Explicitly support this with a throwOnLogLevels configuration property or similar, and document its purpose for test suites, or
• Just add some way to hook into all logs, or all logs of a level. We can document that it might be expensive, and it's mainly intended for test suite features.

[courajs]

Two example tests that are currently logging unexpected errors: https://github.com/cardstack/cardstack/blob/532f62a03e08917d77261552014f9eb4a5bef3f8/packages/ephemeral/node-tests/ephemeral-test.js#L300

They have the additional wrinkle of asserting on the rejection reason of the method that causes the error log. We need to make sure the expectError mechanism doesn't make this too awkward