Add option to prefer movie.nfo when reading/writing nfo files

[randellhodges]

Many years ago, I asked for an option to always use the movie.nfo file instead of the <movie_name>.nfo, both when reading and saving the nfo files. I had something hacked together that I was using, but thought it might be time to try and get something official.

This change adds an option to "Prefer movie.nfo", which, by default is off, so it maintains original behavior.

I pulled some stuff into the helper class and attempted to reuse the same method when finding and saving the nfo files. I also added a Test project just to sanity check some changes. It was really only designed to run on windows/net8, but the purpose of the test project isn't to test all the stuff like IFileSystem or any of the core Emby stuff, just to validate the single helper function I created.

[randellhodges]

I was considering changing MovieInfoSaver.GetSavePath to work a bit like BaseInfoVideoProvider.GetXmlFile, where, it would try to save back to the same file and only use the first candidate if there were no nfo files that already exist.