Previously, like with FileStorage, we were testing btree/go only with
data generated by python and pickle protocol=2. However even on py2
there are more pickle protocols that are in use, and also there is python3.

-> Modernize testdata/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 btree handles all generated zkinds.

All py2_pickle1, py2_pickle2, py2_pickle3 and py3_pickle3 are handled well out of the box.

(*) see nexedi/zodbtools@fac2f190