Add a test for copyright headers for CI

Found a few files that needed copyright headers for a PR I did with:

Get-ChildItem -Recurse . -Include '*.cs' | ? { $_.Name -notlike '*AssemblyInfo*' } | ? { -not (Select-String -Path $_ -Pattern 'Copyright') }

That worked but still returned some false positives.

Given that there's already documentation tests, we should add a copyright header test too -- but it would require some work, since test assets would need to be moved into directories where they won't be looked for...

找到现有的文档测试及其覆盖的测试资源，然后检查 repository 如何发现用于 CI 检查的文件。将所示的 PowerShell 搜索作为了解排除项和误报的上下文。完成的标准是：版权标头测试覆盖预期文件，而无需扫描无关的测试资源。