gio/tests: fix socket /socket/credentials/unix_socketpair on win32

When I enabled unix socketpair test on win32, I left the existing
g_close(fds[1]), but _g_win32_socketpair() returns native sockets
descriptors that must be closed with closesocket() on win32.

Let GSocket handle the socket pair cleanup.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>