Improve draw_tree test suite

[edwardchalstrey1]

At the moment, for the pygambit integration, we just rely on the tests/test_tutorials.py running the tutorial/draw_efg_gambitLayout.ipynb notebook, but it might be better to write more rigorous tests for this.

• Add pytest tests specific to src/core/gambit_layout.py
• Add tests where the gambit object has no player labels (use numbers to denote players "Player 1" etc)
• Ensure multiplier and xshift args cannot cause errors
  • Related to Consider normalising pygambit level spacing options #21
• Include an example of a pygambit game loaded from the OpenSpiel library e.g. tiny hanabi

Additionally, the PNG, PDF (and SVG) generation tests don't currently attempt to actually create images, they just mock and test error raising:

• Ensure image generation from all functions is tested properly
• Update CI to include pre-requisites for image generation so these can be tested properly
• Remove notebook tests completely and refactor everything into the test suite