test: fix metrics api test stability

[piksel]

Should permanently solve the intermittent test failures.

[codecov]

Codecov Report

Merging #930 (1f526d5) into main (b4cf17d) will increase coverage by 14.33%.
The diff coverage is 100.00%.

❗ Current head 1f526d5 differs from pull request most recent head c4a338f. Consider uploading reports for the commit c4a338f to get more accurate results

@@             Coverage Diff             @@
##             main     #930       +/-   ##
===========================================
+ Coverage   40.14%   54.47%   +14.33%     
===========================================
  Files          25       25               
  Lines        1415     1452       +37     
===========================================
+ Hits          568      791      +223     
+ Misses        785      594      -191     
- Partials       62       67        +5     

Impacted Files	Coverage Δ
pkg/notifications/shoutrrr.go	73.43% <100.00%> (+0.42%)	⬆️
internal/actions/update.go	50.47% <0.00%> (-0.55%)	⬇️
pkg/registry/digest/digest.go	0.00% <0.00%> (ø)
pkg/notifications/email.go	90.90% <0.00%> (+4.54%)	⬆️
pkg/container/client.go	14.59% <0.00%> (+14.59%)	⬆️
internal/flags/flags.go	88.47% <0.00%> (+36.89%)	⬆️
pkg/container/container.go	47.31% <0.00%> (+47.31%)	⬆️
pkg/container/metadata.go	63.63% <0.00%> (+63.63%)	⬆️
pkg/container/util.go	100.00% <0.00%> (+100.00%)	⬆️

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[simskij]

I know we found out that the actual reason for the problems was how we do the calls, nevertheless I think the httptest part of this PR is a welcome change as well as it makes the tests less fragile overall. Good job!