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...

找出現有的文件測試及其涵蓋的測試資產，然後檢視 repository 如何探索 CI 檢查所需的檔案。將所示的 PowerShell 搜尋作為了解排除項目與誤報的背景。完成的標準是：版權標頭測試涵蓋預期檔案，而不需要掃描無關的測試資產。