Currently we exercise zodbdump and zodbcommit+zodbdump with non-empty
extensions, which works if ZODB is patched for txn.extension_bytes
support, but fails on pristine ZODB.

Support for txn.extension_bytes cannot get into upstream ZODB for
more than a year:

https://github.com/zopefoundation/ZODB/pull/183
https://github.com/zopefoundation/ZODB/pull/207

and  even if it somehow will make it, it will likely be only in ZODB5,
while we still care to support ZODB4 and ZODB3.

Skipping zodbdump / zodbcommit tests, if a ZODB does not have
txn.extension_bytes support, would result in significant reduction of
zodbtools test coverage, because practically that is the current
situation with all upstream ZODB{3,4,5}. Dropping test coverage for
non-empty extensions is neither a good option.

For those reason, let's rework the tests and test both zodbdump and
zodbcommit with two scenarios:

1. on a test database where transactions extensions are always empty.
   This should work on all ZODB irregardless of whether
   txn.extension_bytes patch is there or not.

2. on a test database where transactions extensions are present.
   This should work if ZODB has txn.extension_bytes support, but if not,
   we can mark this case as xfail, since the failure is expected.

This way we make the testsuite pass irregardless of whether
txn.extension_bytes support is there, and we don't abandon dump/commit
testing coverage.

/helped-by Jérome Perrin <jerome@nexedi.com>