ssh/test: don't use DSA keys in integrations tests, update test RSA key

on RHEL 9 OpenSSH does not support DSA keys and RSA keys with size less
than 2048 bits, furthermore signing with ssh-rsa (SHA-1 signatures) and
ssh-dss is not allowed, therefore:

1) replaced the 1024-bit RSA key used in the test with a new 2048-bit
   one
2) removed DSA key from itegration tests
3) allowed signature errors using ssh-rsa in agent integration tests, we
   also check SHA-2 variants that are not skipped

Fixes golang/go#65581

Change-Id: I54bf997b61ef4d91d38eb624275737ba7291bb20
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/562755
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/test/cert_test.go

@@ -19,7 +19,7 @@ func TestCertLogin(t *testing.T) {
 	s := newServer(t)
 
 	// Use a key different from the default.
-	clientKey := testSigners["dsa"]
+	clientKey := testSigners["ed25519"]
 	caAuthKey := testSigners["ecdsa"]
 	cert := &ssh.Certificate{
 		Key:             clientKey.PublicKey(),

ssh/test/session_test.go

@@ -448,7 +448,6 @@ func TestKeyExchanges(t *testing.T) {
 func TestClientAuthAlgorithms(t *testing.T) {
 	for _, key := range []string{
 		"rsa",
-		"dsa",
 		"ecdsa",
 		"ed25519",
 	} {