tests: add attachement tests integration

[sapk]

Extracted from #7956, to make them work before changing (rebasing) anything.

I haven't very much time lately to do a rebase of the PR in one-shoot. I think this could be already merged as it add more test on current code and it will ease the review later when I rebase the PR with fix.

[codecov-io]

Codecov Report

Merging #9309 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9309      +/-   ##
==========================================
+ Coverage   41.17%   41.19%   +0.02%     
==========================================
  Files         559      559              
  Lines       73101    73101              
==========================================
+ Hits        30096    30116      +20     
+ Misses      39235    39209      -26     
- Partials     3770     3776       +6

Impacted Files	Coverage Δ
models/unit.go	62.16% <0%> (-5.41%)	⬇️
routers/repo/view.go	38.59% <0%> (-0.88%)	⬇️
modules/context/context.go	52.85% <0%> (+0.47%)	⬆️
routers/routes/routes.go	83.25% <0%> (+0.59%)	⬆️
models/release.go	51.13% <0%> (+0.9%)	⬆️
routers/repo/attachment.go	40.67% <0%> (+30.5%)	⬆️

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[guillep2k]

Typo nit. Otherwise LGTM.

[lunny]

wait #9309 (comment)

[sapk]

@lunny done