Infrastructure for integration tests + example test

[kaiboesefeldt]

Branch: requirement_tests
The target is to be able to start a large part of the CovidWatch app in a JUnit test. Those parts of the system, which cannot be run in such a test must be mocked, e.g. GUI, access to BLE, database, backend, must be mocked. Then JUnit tests can be written that trigger events and check results at the mocked system border.
To be able to exchange these components by mocks, they must be instantiated by Koin modules and solely retrieved via Koin. This is the first refactoring step. The target is to be able to run the CovidWatch app in a JUnit test without crash, but just doing nothing.
In a second step, one example integration test shall be written which illustrates how such tests work.