test: Add new acceptance test project

[hkupty]

Reference: #357

So, I've decided to start simple and go with a basic set of tests, with the intention of iteratively going through some cases and getting to a point where this is actually usable by other slf4j implementations.

I also took the chance to implement it for slf4j-simple as means of testing the API.

I went on with your suggestion as it seems to be more fitting to the usual java projects and, although AcceptanceTest doesn't look like a regular test file I'm quite satisfied with how it looks, so I'm curious for your opinion.

I'm keeping this PR as draft for now as I'm of course willing to add more tests and adapt names, project structure and other small details.

Looking forward to hearing your opinion.

[hkupty]

I'm relatively happy with this approach. I'd like to know whether we need more cases for org.slf4j.Logger implementations.

Additional suites should be created after this one for Marker and MDC support I'd say. I'm not sure if those should come in the same PR or in a separate batch.