text: Title: Improve interactions tests
  Body: ## Future works for `tests.test_interactions`
  - make the nested functions (save_screenshot, perform_dragging, etc.) class methods of Test, and initialize the app in setUpClass (inherited from IntegrationTests).
  - break down the interactions tests into separate tests, one for dragging, one for clicking, one for hovering, etc.
  - programmatically set the `renderedPositions` of the nodes, rather than hardcoded.
  
  Those suggestions do not currently affect the testing procedures, but shall we add supplementary tests or make substantial changes to the dash-cytoscape API, they could end up becoming useful. They might take substantial effort to implement.