test: add unit test for dfget

[Lzzzzzt]

Add unit tests for util function in dfget.

Description

This PR is done by finishing the following tasks:

• Add unit tests for validate_args.
• Add unit tests for make_output_by_entry

Related Issue

#496

Motivation and Context

Dragonfly rust client does not support back-to-source downloading of different object storage protocols. Priority is given to supporting OSS protocol back-to-source downloads. Need to support OSS protocol in dragonfly-client-backend crate, implemented based on trait Backend and it needs to support directory recursive downloading.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 25.91%. Comparing base (e2209a8) to head (a2d4b1f).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #748      +/-   ##
==========================================
+ Coverage   23.45%   25.91%   +2.46%     
==========================================
  Files          55       55              
  Lines        7337     7493     +156     
==========================================
+ Hits         1721     1942     +221     
+ Misses       5616     5551      -65     

Files with missing lines	Coverage Δ
dragonfly-client/src/bin/dfget/main.rs	28.02% <100.00%> (+28.02%)	⬆️

... and 2 files with indirect coverage changes

[gaius-qi]

LGTM