.gitignore filtering trumps explicit lists of files

[mparry]

If you specify some files to be copied in a files.source section in a test, this is silently filtered against .gitignore and that can end up overriding the list that you've explicitly given. I expect there may be other instances of this, hence the general issue title.

Of course, this will generally be due to user error - as it was in my case, in fact - but it took a while to figure out what was going on, given the lack of output about the filtering that had occurred.

I think it's fine to do this when considering files to be included in the package, say, and it may be reasonable to do it when a directory is given in files.source rather than an actual file, but I do think something different should happen for files at least - e.g. a warning or error.