test: explicitly test alert grouping (#280)

[blenzi]

• test alert grouping within ALERT_RELAXATION_SECONDS (+ tolerance)
• fix image path in Dockerfile-dev

[ghost]

Nice of you to open a PR :pray: When you're ready and want to get it reviewed, post a comment in this Pull Request with this message: /quack review

[codecov[bot]]

Codecov Report

Merging #281 (20699e5) into main (fa2624d) will increase coverage by 0.22%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
+ Coverage   95.12%   95.35%   +0.22%     
==========================================
  Files          63       66       +3     
  Lines        1497     1571      +74     
==========================================
+ Hits         1424     1498      +74     
  Misses         73       73              

Flag	Coverage Δ
client	100.00% <ø> (?)
unittests	95.12% <ø> (ø)

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see 3 files with indirect coverage changes