go/zodb/zeo: Test it on all py2/py3 ZODB kinds of data we care about and wrt both ZEO/py2 and ZEO/py3

Similarly to FileStorage, fs1tools and other Go packages previously we
were testing ZEO/go client only with old FileStorage testdata generated
by python2 and pickle protocol=2. However even on py2 there are more
pickle protocols that are in use, and also there is python3.

We were also testing our ZEO/go client only wrt ZEO/py2 but there is
also ZEO/py3.

-> Adjust ZEO/go testing to automatically load and test against all ZODB
kinds from recently updated FileStorage testdata and wrt both ZEO/py2
and ZEO/py3.

All py2_pickle1, py2_pickle2, py2_pickle3 and py3_pickle3 are handled well out of the box.
However only ZEO/py2 succeeds: tests wrt ZEO/py3 server currently fail
and so are marked with "xfail".

We will fix tests for ZEO/py3 in the next patch.