Fix the Keyfile Test on Windows

Fix the Keyfile Test on Windows

Windows will not allow one to write to a temp file opened by g_mkstemp()
by opening another fd associated with it before one closes the fd that
is returned by g_mkstemp(), which will cause the test_save test to fail.

Fix this by using a variable to store the fd from g_mkstemp() and checking
it, and call close() on that variable before attempting to call
g_key_file_save_to_file() on the temp file as that will attempt to open
another fd (which would not work) associated with that temp file.

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