Move DummyBackend to openeo.testing

[VictorVerhaert]

this TODO:

openeo-python-client/openeo/rest/_testing.py

Lines 28 to 34 in 0cf3bdd

	class DummyBackend:
	"""
	Dummy backend that handles sync/batch execution requests
	and allows inspection of posted process graphs
	"""
	# TODO: move to openeo.testing

so users can use this for their own testing in a future proof manner

CC @soxofaan