move slf4j implementation to test scope

[Gr33ndev]

sorry, I saw this too late: the library should not force ANY implementation
it's up to the library user to choose what logging implementation he uses:

• Maven uses something
• someone else in another runtime context will choose another implementation

in the context of the library, perhaps setting an implementation with a test scope can make sense

After this discussion in my last PR #185 here is the requested change.

[goneall]

Thanks @Gr33ndev

[Gr33ndev]

You‘re welcome!