Add unit test for pkg/uploader

[qiuming-best]

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Does your change fix a particular issue?

Fixes #(issue)
#6256

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[codecov-commenter]

Codecov Report

Merging #6374 (f74054f) into main (8cd55d1) will increase coverage by 1.56%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #6374      +/-   ##
==========================================
+ Coverage   48.32%   49.88%   +1.56%     
==========================================
  Files         227      227              
  Lines       23376    23376              
==========================================
+ Hits        11296    11661     +365     
+ Misses      11214    10836     -378     
- Partials      866      879      +13     

Impacted Files	Coverage Δ
pkg/uploader/kopia/snapshot.go	79.71% <100.00%> (+47.64%)	⬆️
pkg/uploader/provider/kopia.go	85.03% <100.00%> (+30.61%)	⬆️
pkg/uploader/provider/restic.go	79.41% <100.00%> (+54.90%)	⬆️

... and 3 files with indirect coverage changes