High-excitation code paths in MSD are untested

[prckent]

Describe the bug The work in #3900 highlights that the "general" code path for excitations is untested.

To Reproduce Run the tests.

Expected behavior Unit or deterministic test hits the code path.

Additional context

An easy solution would be to add an MSD only integration test (short and deterministic) using FCI on a pseudoatom. Better would be properly unit testing the general code paths.

In view of the offload work and still-to-be-proven nature of offload compilers, these code paths should be tested.