Fix missing using clause on Disposable Instances for unit tests

[kbeaugrand]

Description

What's new?

• Related to Feature/376 prom custom metrics #845

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Tests
• Other

[codecov]

Codecov Report

Merging #851 (8fb5e4e) into main (22f3a13) will decrease coverage by 0.08%.
The diff coverage is 68.42%.

❗ Current head 8fb5e4e differs from pull request most recent head e53f166. Consider uploading reports for the commit e53f166 to get more accurate results

@@            Coverage Diff             @@
##             main     #851      +/-   ##
==========================================
- Coverage   83.02%   82.93%   -0.09%     
==========================================
  Files         149      149              
  Lines        5225     5234       +9     
==========================================
+ Hits         4338     4341       +3     
- Misses        887      893       +6     

Impacted Files	Coverage Δ
...b.Portal/Client/Pages/Configurations/Configs.razor	96.96% <66.66%> (-3.04%)	⬇️
...ceConfigurations/DeviceConfigurationListPage.razor	88.88% <66.66%> (-2.29%)	⬇️
...lient/Pages/DeviceModels/DeviceModelListPage.razor	93.61% <66.66%> (-1.94%)	⬇️
...Client/Pages/Edge_Devices/EdgeDeviceListPage.razor	67.16% <66.66%> (-1.02%)	⬇️
...es/LoRaWAN/Concentrator/ConcentratorListPage.razor	58.62% <66.66%> (-1.03%)	⬇️
...b.Portal/Client/Pages/Devices/DeviceListPage.razor	68.36% <75.00%> (-0.71%)	⬇️

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