Add tests for Spline tool to support continuing a previously drawn spline

[Sidharth-Singh10]

Closes part of #2483

[Keavon]

If you were planning to continue adding more tests from #2483, please don't start any new ones until #2368 is merged.

Please also avoid opening too many PRs for small sub-tasks in one of those issues that have multiple bullet points, just add them all to the same PR if you can and they're related. It adds a significant amount of overhead dealing with multiple PRs.

[Sidharth-Singh10]

Noted. Shall I close this one or keep it as draft?

[Keavon]

Please finish this one, but after it's ready to merge, don't work on new Spline tool things until after that other issue lands.

[0HyperCube]

Looks good so far.

[Sidharth-Singh10]

@0HyperCube I've refactored the redundant code across the three open PRs.
The assert_point_positions function is reusable and can be shared across tests in all tools.

Would it make sense to include this in EditorTestUtils, or do you suggest a more appropriate place for it?

[0HyperCube]

Looks good; thanks for simplifying the test code.
I think the assert_point_positions is fine in the spline tool as it is useful for that tool only.