gsocket: add a testcase that shows a hang on win32

gsocket: add a testcase that shows a hang on win32

Add a unit test that checks g_socket_new_from_fd by creating
a gsocket, obtaining its fd, duplicating the fd and then creating
a gsocket from the new fd. This shows a hang on win32 since the
gsocket created from the fd never receives the FD_WRITE event
because we wait for the condition without first trying to write
and windows signals the condition only after a EWOULDBLOCK error.

https://bugzilla.gnome.org/show_bug.cgi?id=741707