make the client send a packet when the handshake completes (in gQUIC)

In gQUIC, there's no equivalent to the Finished message in TLS. The
server knows that the handshake is complete when it receives the first
forward-secure packet sent by the client. If the protocol demands that
the server sends the first data, this would never happen. We need to
make sure that the client actually sends such a packet. Queueing a PING
frame is an easy way to do so.

session_test.go

@@ -1470,6 +1470,22 @@ var _ = Describe("Client Session", func() {
 		newCryptoSetupClient = handshake.NewCryptoSetupClient
 	})
 
+	It("sends a forward-secure packet when the handshake completes", func() {
+		sess.packer.hasSentPacket = true
+		done := make(chan struct{})
+		go func() {
+			defer GinkgoRecover()
+			err := sess.run()
+			Expect(err).ToNot(HaveOccurred())
+			close(done)
+		}()
+		close(handshakeChan)
+		Eventually(mconn.written).Should(Receive())
+		//make sure the go routine returns
+		Expect(sess.Close(nil)).To(Succeed())
+		Eventually(done).Should(BeClosed())
+	})
+
 	Context("receiving packets", func() {
 		var hdr *wire.Header