Initialize empty PROM_CONFIG_SUFFIX in test suite

[pinoOgni]

This PR explicitly initializes PROM_CONFIG_SUFFIX variable within the test suite.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.90%. Comparing base (cc5693e) to head (ed99a6e).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1460      +/-   ##
==========================================
- Coverage   43.90%   43.90%   -0.01%     
==========================================
  Files         313      313              
  Lines       34213    34213              
==========================================
- Hits        15022    15020       -2     
- Misses      18213    18214       +1     
- Partials      978      979       +1     

Flag	Coverage Δ
integration-test	21.48% <ø> (-0.27%)	⬇️
integration-test-arm	0.00% <ø> (ø)
integration-test-vm-x86_64-5.15.152	0.00% <ø> (ø)
integration-test-vm-x86_64-6.10.6	0.00% <ø> (ø)
k8s-integration-test	2.29% <ø> (ø)
oats-test	0.00% <ø> (ø)
unittests	44.89% <ø> (ø)

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[rafaelroquetto]

LGTM - out of curiosity, why is this necessary?

[pinoOgni]

LGTM - out of curiosity, why is this necessary?

Some yaml files use PROM_CONFIG_SUFFIX without setting it. It's true that the value is an empty string, but it's more appropriate to set it explicitly, since the file name and therefore the Prometheus configuration changes based on the value.