Is it possible to implement grey-box unit tests for nodes/nodelets with the built-in test infrastructure of ROS2?
A grey-box test is a mixture of a black-box and a white-box test. I would consider e.g. supplying a node with input data via its interface (actions, services, topics, parameters) and assert something in the nodes library level (e.g. if mocked functions in the nodes library level have been called) as node grey-box test. (In a black-box node test the output data is captured via the nodes interface only and asserted in test nodes. In a white-box node test the nodes library level code is verified separate from the nodes interface.)