test: add concurrent download e2e testings

[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 52.81%. Comparing base (2901df0) to head (9a8359a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3212      +/-   ##
==========================================
+ Coverage   52.79%   52.81%   +0.01%     
==========================================
  Files         192      192              
  Lines       20341    20341              
==========================================
+ Hits        10740    10744       +4     
  Misses       8797     8797              
+ Partials      804      800       -4     

Flag	Coverage Δ
unittests	52.81% <ø> (+0.01%)	⬆️

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see 4 files with indirect coverage changes