tests: remove irrelevant assertions from test-aio

In these tests, the purpose of the initial calls to aio_poll and
g_main_context_iteration is simply to put the AioContext in a
known state; the return value of the function does not really
matter.  The next patch will change those return values; change
the assertions to a while loop which expresses the intention
better.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-3-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>