Improve test coverage for SpanExtensions.IndexOf

[Youssef1313]

Closes #<ISSUE_NUMBER>

Ensures that replacing sizeof(T) by Marshal.SizeOf<T>() will cause test failures ;)

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

[Sergio0694]

Thank you! 🙂