This adds a basic set of tests for curve sampling and bevel generation.
At the moment there are basic test cases for bevels, caps, and the
filling of 2D curves, but more tests can be added in the future.
Curves are actually converted to "DispLists" for displaying them in the
viewport, so it's much simpler to rely on the mesh conversion operator
instead of building a new test framework for another data structure.
Differential Revision: https://developer.blender.org/D9958
