[nixio] Increase channel counts in tests
[nixio] Increase channel counts in tests

Increasing number of channels in signals to > 10 to catch issues with
lexicographical ordering of DataArray names during loading (see #545).

JuliaSprenger/python-neo,apdavison/python-neo,INM-6/python-neo,rgerkin/python-neo,samuelgarcia/python-neo,NeuralEnsemble/python-neo