Python wrapper: tests

[akuesters]

Add tests to python/test:

• [ ] test_recipe
• [x] test_domain_decomposition
• [ ] test_cells
  • [ ] test_cable_cell
    • [ ] test_segment
    • [ ] test_synapses
  • [ ] test_lif_cell
  • [ ] test_benchmark_cell
  • [ ] test_spike_source_cell
• [x] test_probes
• [x] test_spikes