We add a program to generate a test database with all fancy features and
then check `zodb dump` output on it to golden on.

The test database itself is commited to git because we want to make sure
zodbdump works ok for particular exact data and because transaction
extension dict is potentially saved differently on various runs.

Quoting

  https://docs.python.org/2.7/library/stdtypes.html#dict.items    and
  https://docs.python.org/3.7/library/stdtypes.html#dictionary-view-objects

""" CPython implementation detail: Keys and values are listed in an arbitrary
    order which is non-random, varies across Python implementations, and depends
    on the dictionary’s history of insertions and deletions. """

This way on test/gen_testdata.py changes it has to be run manually, and
then the output result of the run committed back together with
gen_testdata.py changes.