This is not intended as a _fix_ for #4481, since it's possible that Mocha's behavior in v8.2.0 uncovers false positives.  In other cases--depending on the use-case--they are not false positives at all, but rather annoyances that depended on the pre-v15 behavior of Node.js to only issue warnings.

This PR changes the behavior of Mocha so that it will re-emit unhandled rejections to `process` _if_ they did not generate from Mocha.  If the rejection generated from Mocha, then we treat it as an uncaught exception, because Mocha should not be in the business of ignoring its own unhandled rejections.  The logic for detecting an "error originating from Mocha" is not exhaustive.

Once an unhandled rejection is re-emitted to `process`, Node decides what to do with it based on how it is configured to handle unhandled rejections (strict, warn, quiet, etc.).

Added a public method to `errors` module; `isMochaError()`

Ref: #4481

Signed-off-by: Christopher Hiller <boneskull@boneskull.com>