Add config service unit tests

[kbeaugrand]

Description

What's new?

• Add config service unit tests

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 #223 (f438953) into main (27bb934) will increase coverage by 0.46%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
+ Coverage   11.87%   12.34%   +0.46%     
==========================================
  Files          71       71              
  Lines        2357     2357              
==========================================
+ Hits          280      291      +11     
+ Misses       2077     2066      -11     

Impacted Files	Coverage Δ
...reIoTHub.Portal/Server/Services/ConfigsServices.cs	100.00% <0.00%> (+100.00%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 27bb934...f438953. Read the comment docs.