[Task] Test Coverage Subaccount

[sdischer-sap]

Context
The Subaccount was one of the first resources we added. We learned a lot since then. It could use some refactoring as well as Upgrading Test Coverage before we actually promote it to new api versions.

Scope
As a first step we should fully cover the controller with unittests.
Currently there is still tests missing for:

• Connect()
• Delete()

Stretch Goals
The implementation isn't separated into Controller and Client, which makes it harder to maintain. Eventually we should invest into refactoring here. This can be picked up as part of this task or created as follow up.