x/crypto/ssh: close memPipe after running kex test.

Change-Id: I6022d669946a7cb3403757a7c0dbf21a474e1c95
Reviewed-on: https://go-review.googlesource.com/13590
Reviewed-by: Adam Langley <agl@golang.org>

ssh/kex_test.go

@@ -26,10 +26,12 @@ func TestKexes(t *testing.T) {
 		var magics handshakeMagics
 		go func() {
 			r, e := kex.Client(a, rand.Reader, &magics)
+			a.Close()
 			c <- kexResultErr{r, e}
 		}()
 		go func() {
 			r, e := kex.Server(b, rand.Reader, &magics, testSigners["ecdsa"])
+			b.Close()
 			s <- kexResultErr{r, e}
 		}()