fix: Throw IOException when deleting current directory

[pw-sgr]

dotnet has a handle on the current working directory, this will prevent deletion of any part of it.
Since we have the current working dir in the mock, we can support at least this handle

var fileSystem = new MockFileSystem();
string path = fileSystem.Path.Combine(fileSystem.Path.GetTempPath(), "my-test");
fileSystem.Directory.CreateDirectory(path);
fileSystem.Directory.SetCurrentDirectory(path);
fileSystem.Directory.Delete(fileSystem.Directory.GetCurrentDirectory());
fileSystem.Directory.SetCurrentDirectory(fileSystem.Directory.GetCurrentDirectory());

Expected:
System.IO.IOException : The process cannot access the file '\\?\<Some dir that exists>' because it is being used by another process.

Actual:
Nothing, can delete the current working dir on the mock, but not on the actual file system
Will then throw it can't find the dir on the second SetCurrentDirectory

[vbreuss]

Should I assign this issue to you, @pw-sgr ?

[pw-sgr]

@vbreuss yes, please

[pw-sgr]

We could also add a general handle mocking, but that would blow the scope of this issue. In addition this feature would open a whole new can of worms on how everything is handled and would need a new public mocking api, but it's worth the to think about it