Metrics engine tests

[codebien]

#2815 (comment)

// TODO: refactor and move the tests that still make sense in other packages

It resolves the unit tests parts

[codecov-commenter]

Codecov Report

Merging #2878 (9df40a0) into remove-the-engine (27b1a4d) will decrease coverage by 0.19%.
The diff coverage is 80.39%.

❗ Current head 9df40a0 differs from pull request most recent head 5847bdc. Consider uploading reports for the commit 5847bdc to get more accurate results

@@                  Coverage Diff                  @@
##           remove-the-engine    #2878      +/-   ##
=====================================================
- Coverage              76.75%   76.56%   -0.19%     
=====================================================
  Files                    224      222       -2     
  Lines                  16835    16765      -70     
=====================================================
- Hits                   12921    12836      -85     
- Misses                  3080     3094      +14     
- Partials                 834      835       +1     

Flag	Coverage Δ
ubuntu	76.56% <80.39%> (-0.05%)	⬇️
windows	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/ui.go	68.87% <ø> (ø)
cmd/run.go	70.61% <75.43%> (ø)
output/manager.go	84.09% <91.66%> (ø)
metrics/engine/engine.go	88.88% <100.00%> (+3.54%)	⬆️
loader/filesystems.go	60.00% <0.00%> (-40.00%)	⬇️
js/common/initenv.go	66.66% <0.00%> (-33.34%)	⬇️
api/v1/metric.go	54.54% <0.00%> (-13.64%)	⬇️
js/modulestest/modulestest.go	66.66% <0.00%> (-11.12%)	⬇️
js/modules/k6/experimental/tracing/client.go	61.11% <0.00%> (-7.76%)	⬇️
api/v1/status_routes.go	53.06% <0.00%> (-6.13%)	⬇️
... and 22 more

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