fix a race condition in a session test

We were relying on the run-loop to set handshakeComplete to true, while
at the same time sending packets. select doesn't guarantee the order of
execution when multiple cases can run. The fix is setting
handshakeComplete to true directly.

session_test.go

@@ -1377,7 +1377,7 @@ var _ = Describe("Session", func() {
 		})
 
 		It("ignores undecryptable packets after the handshake is complete", func() {
-			close(aeadChanged)
+			sess.handshakeComplete = true
 			go sess.run()
 			sendUndecryptablePackets()
 			Consistently(sess.undecryptablePackets).Should(BeEmpty())