Fix/add unitary tests

[kbeaugrand]

Description

What's new?

• Add unitary test on gateway creation

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 #221 (7657883) into main (6b677ce) will increase coverage by 1.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   10.26%   11.28%   +1.01%     
==========================================
  Files          71       71              
  Lines        2357     2357              
==========================================
+ Hits          242      266      +24     
+ Misses       2115     2091      -24     

Impacted Files	Coverage Δ
...ub.Portal/Server/Controllers/GatewaysController.cs	27.95% <0.00%> (+5.59%)	⬆️
src/AzureIoTHub.Portal/Shared/Models/Gateway.cs	55.55% <0.00%> (+55.55%)	⬆️
src/AzureIoTHub.Portal/Shared/Models/ConfigItem.cs	71.42% <0.00%> (+71.42%)	⬆️

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