Refactor MixProducer test

[Gsantomaggio]

Fixes #81

[codecov-commenter]

Codecov Report

Merging #82 (891c7eb) into main (850db94) will increase coverage by 0.00%.
The diff coverage is 97.82%.

❗ Current head 891c7eb differs from pull request most recent head 12f3b07. Consider uploading reports for the commit 12f3b07 to get more accurate results

@@           Coverage Diff           @@
##             main      #82   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files          69       69           
  Lines        4502     4535   +33     
  Branches      262      265    +3     
=======================================
+ Hits         4087     4117   +30     
  Misses        358      358           
- Partials       57       60    +3     

Impacted Files	Coverage Δ
Tests/ProducerSystemTests.cs	99.26% <97.82%> (-0.74%)	⬇️
RabbitMQ.Stream.Client/Subscribe.cs	63.73% <0.00%> (-1.10%)	⬇️
RabbitMQ.Stream.Client/MetaData.cs	90.65% <0.00%> (-0.94%)	⬇️

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