testutil/compose: add basic prometheus alerts

[corverroos]

Adds a few basic alerts to compose prometheus. Note this is pure prometheus alerts, alertmanager hasn't been configured since it isn't required to group or silence or manage alerts over time.

category: test
ticket: #631

[codecov]

Codecov Report

Merging #719 (a1828dc) into main (62b3975) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #719      +/-   ##
==========================================
+ Coverage   53.97%   54.03%   +0.05%     
==========================================
  Files         104      104              
  Lines       10162    10162              
==========================================
+ Hits         5485     5491       +6     
+ Misses       3871     3867       -4     
+ Partials      806      804       -2     

Impacted Files	Coverage Δ
core/leadercast/transport.go	75.14% <0.00%> (-1.19%)	⬇️
core/aggsigdb/memory.go	78.89% <0.00%> (+0.91%)	⬆️
dkg/dkg.go	52.90% <0.00%> (+1.62%)	⬆️

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