Revise validation strategy for conformance of API and CLI

[ckunki]

Currently ITDE contains automated tests verifying the compatibility of API and CLI.
However the current approach creates some problems.

Problem1: There are two API branches, while only the second one is verified.

lib/api/spawn_test_environment_with_test_container.py

@no_cli_function
def spawn_test_environment_with_test_container(

lib/api/spawn_test_environment:

@cli_function
def spawn_test_environment()

Problem 2: Is 100% identical interface really desired or required?

Potential situations uncovered by the current approach:

• API could offer more functionality than what should be accessible via CLI
• API may benefit for more sophisticated data types, e.g. PortForwarding rather than only basic types such as str and int than can be supplied via CLI.