builder: add some unit test cases

[hijackthe2]

Some unit test cases are added for compact.rs, lib.rs, merge.rs, stargz.rs, core/context.rs, and core/node.rs in builder/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 #1467 (c015e3e) into master (689900c) will increase coverage by 8.13%.
The diff coverage is 95.82%.

@@            Coverage Diff             @@
##           master    #1467      +/-   ##
==========================================
+ Coverage   45.65%   53.79%   +8.13%     
==========================================
  Files         123      123              
  Lines       39170    40172    +1002     
  Branches    39170    40172    +1002     
==========================================
+ Hits        17885    21610    +3725     
+ Misses      20299    17336    -2963     
- Partials      986     1226     +240     

Files	Coverage Δ
builder/src/core/context.rs	67.03% <100.00%> (+23.03%)	⬆️
builder/src/lib.rs	64.79% <100.00%> (+12.02%)	⬆️
builder/src/merge.rs	71.33% <100.00%> (+71.33%)	⬆️
builder/src/stargz.rs	74.67% <100.00%> (+74.67%)	⬆️
builder/src/core/node.rs	82.47% <99.47%> (+40.93%)	⬆️
builder/src/compact.rs	80.56% <92.49%> (+80.56%)	⬆️

... and 22 files with indirect coverage changes