[camunda-mockito] Provide possibility to register embedded subprocess mocks

[jangalinski]

(Kopie von camunda-community-hub/camunda-platform-7-mockito#42)\nOriginal: https://github.com/camunda-community-hub/camunda-platform-7-mockito/issues/42\n\n---\n\nRequirement: When Testing a process with an embedded subprocess, it should be possible to perform the test without defining delegates of the subprocess. The subprocess should get mocked and just return an pre defined return value.

Acceptance criteria/MVP:

• Provide possibility to register embedded subprocess mocks and define its output.
• It should also be possible to verify if the embedded subprocess was executed.
• the sub-process has to be cleaned up after the test ran (managed by the ProcessEngineRule)

Additional behaviours:

• Process may be started synchroniously/asynchroniously
• Run into an Error
• Send a message
• Wait for a message receive
• Wait for a timer