Test corner cases

[tomalrussell]

Some corner cases to test:

• vertical and horizontal lines, including collinear with grid lines, e.g. (0.5, 1) to (3, 1)
• lines that start and/or end on grid lines at a point, including at grid corners
• Test raster intersection with line segment external to grid #47

It would be nice to have a parameterised test or helper that can take a linestr, grid and expected splits, so we can quickly write a bunch of cases. Maybe generators in Catch2 terminology? If this is straightforward, bundle it in with work on this issue. Otherwise a little boilerplate is fine.

[tlestang]

Would be good to be able to know which test case(s) fail when the test runner report an error.