The patch that provides raw-extension functionality was merged into ZODB 5.6:

https://github.com/zopefoundation/ZODB/commit/2f8cc67a3ba3

So when testing with ZODB5 >= 5.6 the tests will excercise code path
that uses txn.extension_bytes, and when testing with ZODB4 the tests
will excercise code path that work-arounds lack of txn.extension_bytes.

/reviewed-by @jerome
/reviewed-on nexedi/zodbtools!18