storage: add some unit test cases

[hijackthe2]

Some unit test cases are added for device.rs, meta/batch.rs, meta/chunk_info_v2.rs, meta/mod.rs, and meta/toc.rs in storage/src to increase code coverage.

Relevant Issue (if applicable)

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Details

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Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• 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

Go over all the following points, and put an x in all the boxes that apply.

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

[codecov]

Codecov Report

Merging #1469 (d4f688f) into master (8b81a99) will increase coverage by 1.05%.
The diff coverage is 99.02%.

@@            Coverage Diff             @@
##           master    #1469      +/-   ##
==========================================
+ Coverage   53.80%   54.85%   +1.05%     
==========================================
  Files         123      123              
  Lines       40172    40477     +305     
  Branches    40172    40477     +305     
==========================================
+ Hits        21613    22204     +591     
+ Misses      17333    17034     -299     
- Partials     1226     1239      +13     

Files	Coverage Δ
builder/src/stargz.rs	74.67% <100.00%> (ø)
storage/src/device.rs	67.89% <100.00%> (+0.93%)	⬆️
storage/src/meta/batch.rs	97.70% <100.00%> (+97.70%)	⬆️
storage/src/meta/chunk_info_v2.rs	97.68% <100.00%> (+26.63%)	⬆️
storage/src/meta/mod.rs	67.88% <100.00%> (+5.58%)	⬆️
storage/src/meta/toc.rs	70.86% <96.73%> (+14.85%)	⬆️

... and 3 files with indirect coverage changes