Since 0b6f99da (test/gen_testdata: Fix for ZODB5 > 5.5.1 + preserve
database compatibility with ZODB3/py2) we are generating our test
database with using pickle protocol=2. This was done in order to make
sure the zodbtools works ok with data generated by e.g. ZODB4. However
we still have lots of databases that are generated with pickle
protocol=1, and we also have newer databases that are generated with
pickle protocol=3. Note that it is not only data records who are
affected by specified pickle protocol. For example for FileStorage the
index is also saved via pickling and so used pickle protocol affects
index format. For example ZODB/go currently cannot load FileStorage
index if it was saved via protocol=3.

Since zodbtools should work ok with any data it creates a need to test
it against all kinds of ZODB databases: generated be either py2 or py3,
and saved via pickle protocol 1,2 and 3.

In this patch we add infrastructure for such testing and extend testdata
coverage to cover not only py2_pickle2, but also py2_pickle1. We will
add support for py2_pickle3 and py3_pickle3 in the follow-up patches.

Testdata files are now located inside dedicated subdirectories - one for
one ZODB kind. py2_pickle2/* is exactly the same compared to testdata
files we had before. py2_pickle1/* is bit different. It is handy to see
the difference via e.g.

    $ diff -u py2_pickle2/zdump.zpickledis.ok py2_pickle1/zdump.zpickledis.ok

In tests particular kind of testdata is now accessed via ztestdata
fixture. Please see changes in zodbtools/test/conftest.py and
zodbtools/test/gen_testdata.py for details.