Backport tests of the Python client library

[flavray]

The Avro Python client library has an extensive suite of tests to ensure encoding, decoding, schema reading, etc... are working well.

Let's try and backport them as part of an "integration" test suite for avro-rs

https://github.com/apache/avro/tree/master/lang/py/test

[poros]

@flavray do you want to add it to the first milestones or is it more of a nice to have?

[flavray]

I kept it out of M1 for now, I think it'd be better to focus on the other issues first.
Don't think this would block the release of M1, but definitely a nice to have though. We'll see how progress is going. :)

[poros]

I had a look at the tests and the vast majority depends on features of the python library that I didn't even know existed (irpc, protocols, tether, etc).

The few ones that are still interesting are https://github.com/apache/avro/blob/master/lang/py/test/test_io.py and https://github.com/apache/avro/blob/master/lang/py/test/test_schema.py.

[poros]

I found a good bunch of test failing which highlighted both bugs and incomplete support of the spec (at least in the areas we are interested in supporting). I opened a bunch of issues to track them.

Together with some regression testing coverage, discovering these issues is probably the best thing we can get out of this whole backporting exercise.