coroutines, testsuite: Fix co-ret-17-void-ret-coro.C.

coroutines, testsuite: Fix co-ret-17-void-ret-coro.C.

This was a bad testcase, found with fsanitize=address; the final suspend
is 'suspend never' which flows off the end of the coroutine destroying
the promise and the frame.   At that point access via the handle is an
error.  Fixed by checking that the promise is destroyed via a global var.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/torture/co-ret-17-void-ret-coro.C: Check for
	promise destruction via a global variable.