g_file_copy: Don't set GError when we intend to ignore errors

For better or worse, the current g_file_copy intention was to ignore
errors copying metadata, but we still set the GError, while returning
TRUE.

https://bugzilla.gnome.org/show_bug.cgi?id=727559
gio/gtestdbus.c: Fix write_config_file() for Windows

Windows does not like g_unlink() to be called on files whose file
descriptor is still open, so doing that would cause a permission
denied error.  Since the fd is not used in that function after
acquiring the temp file, close it earlier before
g_file_set_contents(), so that it can complete successfully.

This fixes a number of GTK+ tests on Windows.

https://bugzilla.gnome.org/show_bug.cgi?id=719344