test: add piece lenght, tag, application test cases for dfget downloading

[gaius-qi]

Description

Related Issue

dragonflyoss/client#398

Motivation and Context

Screenshots (if appropriate)

Types of changes

• 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

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.00%. Comparing base (3a4a053) to head (44a2e39).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3207      +/-   ##
==========================================
+ Coverage   51.96%   52.00%   +0.04%     
==========================================
  Files         173      173              
  Lines       19382    19382              
==========================================
+ Hits        10072    10080       +8     
+ Misses       8601     8595       -6     
+ Partials      709      707       -2     

Flag	Coverage Δ
unittests	52.00% <ø> (+0.04%)	⬆️

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see 2 files with indirect coverage changes