[WIP] Optimize test workflow to run tests in parallel and change to NUnit

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

1/ Test workflow run too long which takes 2min 30s in https://github.com/devstress/My3DLearning/actions/runs/23846716524/job/69517454961?pr=18
Try to run tests per solution in parallel but isolated, instead rehost aspire every time.
2/ Change to nunit test to avoid the following warnings:
Warning:
The component "Fluent Assertions" is governed by the rules defined in the Xceed License Agreement and
the Xceed Fluent Assertions Community License. You may use Fluent Assertions free of charge for
non-commercial use only. An active subscription is required to use Fluent Assertions for commercial use.
Please contact Xceed Sales mailto:sales@xceed.com to acquire a subscription at a very low cost.
A paid commercial license supports the development and continued increasing support of
Fluent Assertions users under both commercial and community licenses. Help us
keep Fluent Assertions at the forefront of unit testing.
For more information, visit https://xceed.com/products/unit-testing/fluent-assertions/

3/ Index the repo to avoid this warning: GitHub Code Search cannot be used because the repository is not indexed
GitHub Code Search tool could not be created (repo is not indexed)!