DRY give_unallocated_credits_for! spec, by stubbing calculate_unallocated_credits

For spec file spec/models/zuora/local/rate_plan_charge_spec.rb refactor.

The following discussion from !14129 should be addressed:

• @cwiesner started a discussion: (+2 comments)

  thought (non-blocking): With this method being public now, do you think it's worth revisiting the specs for give_unallocated_credits_for! and remove this part of the testing there? Some stub could be used instead if possible to simplify those specs. WDYT?