[MRG] fix GUI MPI test (#868)

* test: added assertion that the dipole list is not empty in test_gui_run_simulation_mpi

Previously this test was passing with an empty dipole list. It was not checking any dipoles were actually created.

* test: changed test_gui_run_simulation_mpi to use the default network

-The test fixture network was not working with MPI. Presumably because the smaller network used in the fixture was not compatible with the multiprocessing subprocess call.

-We were encountering an error at the h.finitialize step of network_builder > _simulate_single_trial when using the fixture network. The error was within neuron: "NEURON: mindelay is 0 (or less than dt for fixed step method)".

hnn_core/tests/test_gui.py

@@ -373,16 +373,22 @@ def test_gui_init_network(setup_gui):
 
 @requires_mpi4py
 @requires_psutil
-def test_gui_run_simulation_mpi(setup_gui):
+def test_gui_run_simulation_mpi():
     """Test if run button triggers simulation with MPIBackend."""
-    gui = setup_gui
+    gui = HNNGUI()
+    _ = gui.compose()
 
+    gui.widget_tstop.value = 70
+    gui.widget_dt.value = 0.5
     gui.widget_backend_selection.value = "MPI"
+    gui.widget_ntrials.value = 2
     gui.run_button.click()
+
     default_name = gui.widget_simulation_name.value
     dpls = gui.simulation_data[default_name]['dpls']
     assert isinstance(gui.simulation_data[default_name]["net"], Network)
     assert isinstance(dpls, list)
+    assert len(dpls) > 0
     assert all([isinstance(dpl, Dipole) for dpl in dpls])
     plt.close('all')