fix: use split = false system for remake autodiff tests

[AayushSabharwal]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

why split=false?

[AayushSabharwal]

Two reasons:

1. The tests were written assuming a parameter vector. They actually replaced the MTKParameters object with an Array via remake. The test working was pure coincidence with MTK internals and because the MTKParameters object only had a tunables vector.
2. Gradient w.r.t parameters not working with MTKParameters SciMLSensitivity.jl#1130