Previously we were testing FileStorage/go only with data 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.

-> Modernize py/gen-testdata to use run_with_all_zodb_pickle_kinds
   that was recently added as part of nexedi/zodbtools@f9d36ba7
   and generate test data with both python2 and python3. It is handy to
   use py2py3-venv(*) to prepare python environment to do that.

   Adjust tests on Go side to verify how FileStorage handles all generated zkinds.

py2_pickle1, py2_pickle2 and py2_pickle3 are handled well.
Tests for py3_pickle3 currently fail and so are marked with "xfail".

We will fix tests for py3_pickle3 in follow-up patches.

Old testdata are not yet removed because e.g. fs1tools and zodbdump
tests depend on them. We will remove old fs1 testdata after adjusting
tests in dependent packages step-by-step.

(*) see nexedi/zodbtools@fac2f190