[Experimental] Fix regression tests in test_vectorized_mobject.py

[fmuenkel]

Overview: What does this pull request change?

• Refactors the regression test to check incomplete curves for the OpenGL renderer, which uses 3 point (quadratic) bezier curves instead of Cairo's 4 point (cubic) curves. Reames the test function test_align_points_handles_vmobject_with_no_complete_cubic_curves to test_align_points_handles_vmobject_with_no_complete_curves

• Make Iterable available at runtime, which fixes test_vgroup_init_with_iterable.

Motivation and Explanation: Why and how do your changes improve the library?

The test treated three point curves as incomplete, which is not the case for the OpenGL renderer.

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested