Documentation for Test Concept

[Weltraumschaf]

Acceptance Criteria:

• New "Test Concept Guide" for secureCodeBox is documented here: https://docs.securecodebox.io/docs/contributing/test-concept
• Document where to implement / find unit-tests, integration-tests
• Document how to use/start unit-tests, integration-tests with make

[SebieF]

See also #71.

[Weltraumschaf]

Looks like this is a duplicate of #71. Since #71 has a more detailed story format, I would prefere #71 instead of this issue.