Merge branch 'cleanuptests' into 'master'

test/conftest.py

test/fixtures/operation_tree.py

test/fixtures/port.py

test/fixtures/signal.py

test/fixtures/signal_flow_graph.py

test/test_core_operations.py

test/test_fast_simulation.py

test/test_graph_id_generator.py

test/test_gui.py

test/test_inputport.py

test/test_operation.py

test/test_outputport.py

test/test_schedule.py

test/test_scheduler_gui.py

test/test_sfg.py

test/test_signal.py

test/test_simulation.py