Spline unit tests with large number of splines needed

[prckent]

Unit test needed similar to test_MO with large numbers of (fake) splines to catch errors such as #4439 . See discussion in #4443 . The number of splines should be significantly larger than all of the chunk/batch sizes used internally.

[jptowns]

Having some way of constructing a desired spline type without a builder would also be helpful for unit testing spline rotations.