Add some unit test for utils, storage, rafs, api and builder.

[ZouTaooo]

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Details

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Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1475 (c0350c0) into master (89882a4) will increase coverage by 7.63%.
Report is 1 commits behind head on master.
The diff coverage is 99.81%.

@@            Coverage Diff             @@
##           master    #1475      +/-   ##
==========================================
+ Coverage   54.86%   62.49%   +7.63%     
==========================================
  Files         123      123              
  Lines       40477    43132    +2655     
  Branches    40477    43132    +2655     
==========================================
+ Hits        22208    26956    +4748     
+ Misses      17033    14868    -2165     
- Partials     1236     1308      +72     

Files	Coverage Δ
rafs/src/fs.rs	34.73% <100.00%> (+16.09%)	⬆️
rafs/src/metadata/cached_v5.rs	81.06% <100.00%> (+22.53%)	⬆️
rafs/src/metadata/chunk.rs	88.20% <100.00%> (+34.04%)	⬆️
rafs/src/metadata/direct_v6.rs	44.52% <100.00%> (+5.79%)	⬆️
rafs/src/metadata/inode.rs	95.39% <100.00%> (+26.84%)	⬆️
rafs/src/metadata/layout/v6.rs	85.19% <100.00%> (+13.73%)	⬆️
rafs/src/metadata/md_v5.rs	29.41% <100.00%> (+14.61%)	⬆️
rafs/src/metadata/mod.rs	70.99% <100.00%> (+14.67%)	⬆️
rafs/src/metadata/noop.rs	91.42% <100.00%> (+79.66%)	⬆️
rafs/src/mock/mock_chunk.rs	94.44% <100.00%> (+94.44%)	⬆️
... and 20 more

... and 14 files with indirect coverage changes

[imeoer]

Great work, thanks! @ZouTaooo
cc @jiangliu @adamqqqplay Let's take a look together.

[adamqqqplay]

@ZouTaooo Thanks for your contribution! It looks really good.

[imeoer]

Maybe move it to [dev-dependencies].

[ZouTaooo]

Thanks for your review. Now, I have fixed it with new commit.

[ZouTaooo]

The Smoke-Test found a test-case error. And the commit fix it.
Two seconds is an edge-case, but there is a resolution problem in linux timer. So the data will be updated with different index sometimes. So the new test case with two changes, first using the normal argument rather than edge case, secondly considering the timer delta error.