Improve get all files in directory unit test

[ld-kerley]

As reported in #2663 the get all files in directory unit test is not robust enough to OS generated files.

This PR inverts the validation logic to make it more robust.

Looks like this idea fails the windows CI - but im not sure why and don't have a windows machine to investigate.

[kwokcb]

There is an issue with the logic in FilePath::getFilesInDirectory() for Windows:

You need this change since on FindFirstFileA will try to match that specific file (not the contents of the folder).

    FilePath query = extension.empty() ? (*this / "*") : (*this / ("*." + extension));
    //FilePath query = extension.empty() ? *this : (*this / ("*." + extension)); // finds nothing
    HANDLE hFind = FindFirstFileA(query.asString().c_str(), &fd);

Also suggest to put in a REQUIRE(results.size() > 0) in the unit test to check for this.

I ran with these changes locally and it seems to work if you have extra files in the folders being checked.

[kwokcb]

Thanks for the fix. Looks good.