Generalize encoding/decoding tests

[thake]

The current unit tests for encoding have been refactored as part of #160. Duplicate tests have been removed, and some bugs have been fixed on the way.

[Chuckame]

Let's merge it as it is, as I'm currently adding ktlint, spotless and reformat everything, and also moving forward on the v2, so having simpler tests will be awesome.

I'm not a fan of the open door of multiple EnDecode implementation as it complexifies for a need that we don't have now, and mostly avro4k should not have multiple way of doing avro or users won't really know what to chose.

If we want to test decoding and encoding in multiple container types (single object, confluent schema registry, ...), let's tackle some "wrapping" api to be able of decorate a binary avro message with whatever we need.

I'll do a little other PR to fix this point after all the code reformat