test: repeatability and refactoring

[piksel]

This replaces all usage of Random in the tests with deterministic pseudo random data that should yield the same coverage regardless of runs and if new tests are added (which was the case before).

It also refactors and homogenize the tests a bit, with clearer asserts and labels for non-obvious arguments.

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.

[codecov]

Codecov Report

Merging #671 (a55f226) into master (b5b1a92) will increase coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   73.07%   73.31%   +0.24%     
==========================================
  Files          68       68              
  Lines        8305     8305              
==========================================
+ Hits         6069     6089      +20     
+ Misses       2236     2216      -20     

Impacted Files	Coverage Δ
...ode.SharpZipLib/Zip/Compression/DeflaterHuffman.cs	97.87% <0.00%> (+6.07%)	⬆️

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