Things to test include:

1. MongoDB 4.2 (FCV 4.0) and MongoDB 4.0 allow reading and deleting long (unique) index keys but does not allow inserting or updating long (unique) index keys.
2. MongoDB 4.2 (FCV 4.2) allows inserting, reading, deleting and updating long (unique) index keys.
3. MongoDB 4.0 throws warning on startup when detecting the new feature tracker bit.
4. MongoDB 4.2 could successfully validate index consistency after reindex() if 4.0 intentionally missed out long index keys by setting 'failIndexKeyTooLong' to false.
5. MongoDB 4.0 could successfully validate index consistency after drop() if 4.2 inserted long index keys (with < 127 bytes typebits).