DirectoryIsValid() ignored try_to_create_if_not_exists and always
called mkpath(). On the Windows CI runner (which may create dirs at
the drive root) PathWidget validation both created bogus directories
and never flagged invalid paths; on Linux the mkpath just failed.
- filefunctions: only mkpath when try_to_create_if_not_exists is set
- TaskProjectLoadTest: Project::set_filename() stores native
separators on Windows, normalize before comparing paths
- DialogProjectProperties: use a nonexistent file inside a temp dir
instead of a fixed /definitely/... path that prior tests may have
partially created on a writable drive
Mike-Solar