ssh: add integration tests against a reference implementation

As in crypto/tls, we record the connection against a reference
implementation, OpenSSH in our case, and run part of our integration
tests by replaying these recorded SSH connections.

Change-Id: If042b5f650b267bd3ede34a05ec3a6fa6d1a86b3
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/644436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

Author: drakkan

ssh/test/banner_test.go

@@ -94,7 +94,7 @@ func testPortForward(t *testing.T, n, listenAddr string) {
 	if len(sent) != len(read) {
 		t.Fatalf("got %d bytes, want %d", len(read), len(sent))
 	}
-	if bytes.Compare(sent, read) != 0 {
+	if !bytes.Equal(sent, read) {
 		t.Fatalf("read back data does not match")
 	}