Producer: Rewrite non enabled unit tests

[cbrand]

Description

During the implementation of #270 I realized that a couple of unit tests inside of the tests/unit/transport/test_producer.py doesn't do anything due to early return statements.

This PR adjusts all the tests to check for expected behavior which did not run correctly before.

Open Question is the flush_atmost function. Due to the event set wait method, it cannot be guaranteed that during the flush operation that there are at max the max_messages of sent events being sent. According to the function name and the previous test, however, this is the intended behavior.

I adjusted the test accordingly and just expect that messages are sent out, as there doesn't seem to be a reasonable primitive to fulfill the existing expectation within the test.

[cbrand]

@patkivikram hopefully fixed the formatting issues (i only ran black on the faust repository and not the tests repository accidentally).

[patkivikram]

@cbrand can you resolve the conflicts?

[cbrand]

Yes, will do and I will get back to you.

[codecov-commenter]

Codecov Report

Merging #272 (34ba377) into master (784bee7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #272   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files         100      100           
  Lines       10842    10842           
  Branches     1481     1481           
=======================================
  Hits        10269    10269           
  Misses        500      500           
  Partials       73       73           

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[cbrand]

@patkivikram done. Please check.

I also realized that it seems that the ThreadedProducer isn't tested at all as it would currently fail in Python 3.6 (as it uses asyncio.create_task). See: #277 for a potential solution there.