chore(test): Refactor Core directory in the Crowdin.Api.Tests

[EvilKot]

Closes #272

Reaming folder with test utilities to Testing/ instead of /Core to not to conflict with folder structure defined in /src.

Should be good enough.
Alternatively all tests could be moved into nested directory to avoid further folder conflicts, or even all test utilities and resources could be moved to a new project.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.38%. Comparing base (f45d418) to head (dfb5b9b).
Report is 113 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #274       +/-   ##
===========================================
+ Coverage   50.38%   63.38%   +13.00%     
===========================================
  Files         277      399      +122     
  Lines        3688     6084     +2396     
  Branches        0      501      +501     
===========================================
+ Hits         1858     3856     +1998     
- Misses       1830     2189      +359     
- Partials        0       39       +39     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[andrii-bodnar]

@EvilKot, thank you! Passing it over to @innomaxx for review as well.

[innomaxx]

@andrii-bodnar @EvilKot nice work, but I suggest a slightly different approach.

To refactor the namespaces, we can make the following changes:

1. Rename the project Crowdin.Api.Tests to Crowdin.Api.UnitTesting (it looks better IMO).
2. Move all tests to Crowdin.Api.UnitTesting.Tests.{ApiSectionName} folders
   Core serialization and other tests place in root of Crowdin.Api.UnitTesting.Tests
3. Move all resources to Crowdin.Api.UnitTesting.Resources folder.
4. Move helper and utility classes to Crowdin.Api.UnitTesting folder.