Adds test data and description

[andrewphilipsmith]

This PR provides an initial attempt to add some failing test cases for scenarios where the Inspire polygons need to be classified as road space or not and where non-registered land cannot be assumed to be road space. A qualitative description of each scenario is given in test_cases.md

Included in this PR:

• Provides example data for scenarios where entire polygons need to be classified (Cases 1, 3, and 5).
• Provides example data for a scenario where unregistered land is non-road space and where a classifier would need to separate it from unregistered road-space (Case 2).
• Provides example data for a scenario where two neighbouring registered land polygons both incorporate road-space and non-road-space, and a classifier will need to subdivide them (Case 4).
• Provides a failing test for scenarios where whole land polygons need to be classified (e.g. without subdivision) - (Cases 1, 3, and 5).
• Provides a failing test for scenarios where land registry data (either registered or unregistered land) need to be subdivided to provide a useful classification. (Cases 2, and 4).

[andrewphilipsmith]

This is not urgent, but I think that it would be helpful to review this PR at this stage.

A few thoughts:

• I'm not sure the current mix of how the test data and expected results are stored is ideal. (eg the source data is in small files and the expected results are a mix of small files, or hardcoded into the test code).
• I've not given much thought to how the failing implementation classify_polygons_by_id fits in with the rest of the code yet.
• The second failing test could be added later, either as part of the PR, or a separate future one.