Execute graph without GUI with the Orange engine

[woutdenolf]

When running workflows in unit-tests of ewoksorange and other ewoks-orange apps, I typically run workflows in two ways:

from ewokscore import execute_graph
from ewoksorange.gui.workflows.owscheme import ows_to_ewoks

Example: https://gitlab.esrf.fr/workflow/ewoksapps/ewoksfluo/-/blob/main/src/ewoksfluo/tests/tutorials/assert_results.py

Make a fixture that takes care of the boilerplate or even better, add an option to execute_graph(..., gui=False).