Project DataStructures.Tests - Switch to file-scoped namespaces

[gmottajr]

Project DataStructures.Tests

Switch to file-scoped namespaces
Issue 425 - Migrate to DotNet 8

Motivation and Context
Github issue #425: Proposed to update to File-scoped namespaces. It was requested to perform the changes in a few PRs to make them smaller and easier to review. This is the one only meant to Switch to file-scoped namespaces of the project DataStructures.Tests.

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e8ca3f4) 94.50% compared to head (2e20e1b) 94.41%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
- Coverage   94.50%   94.41%   -0.10%     
==========================================
  Files         234      234              
  Lines        9843     9843              
  Branches     1455     1455              
==========================================
- Hits         9302     9293       -9     
- Misses        381      387       +6     
- Partials      160      163       +3     

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[siriak]

Looks good, thanks!