Add tests for find_decompressed_size with skippable frames

[nhz2]

The implementation of find_decompressed_size is missing the special checks for skippable frames found in zstd

This PR adds tests to ensure the current implementation works correctly with skippable frames.

This PR also fixes initialization problems in some of the tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.55%. Comparing base (a1fe6df) to head (43988dd).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #68   +/-   ##
=======================================
  Coverage   61.55%   61.55%           
=======================================
  Files           5        5           
  Lines         372      372           
=======================================
  Hits          229      229           
  Misses        143      143           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mkitti]

How did you generate the skippable frames? Did you use the static API? If so, I made a module with the static API available for testing. Perhaps you should include code to generate the skippabke frames.

[nhz2]

I made the skippable frame by looking at the output of the pzstd, and from the description in https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#skippable-frames

[mkitti]

There is also ZSTD_writeSkippableFrame from the static API.

I think we might want to consider a generational step where we use the static API to generate the bytes just so it is clear where they came from.

For that reason, I kept the static API around here:

https://github.com/JuliaIO/CodecZstd.jl/blob/master/test/LibZstd_clang_static.jl