Add a test for copyright headers for CI

[rjmholt]

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...