Separate server, parse and store functions from TestBase

[nettle]

Describe the task

Separate server, parse and store functions from TestBase.
Reason: do not mix simple and complex functionality, but separate and extend.

Scope

1. Add a new unittest base class with server, parse and store functionality testing
2. Use new class only in unittest which requires server, parse, store
3. Make sure nothing is broken

Definition of Done

• Separate unittest class with server, parse and store functionality
• Nothing is broken