Render bad RSA key useless for signing by setting modulus

In more recent Go versions requirements for RSA keys have been
strengthened, resulting in errors when trying to generate insecure
keys. In this specific test case we need a bad RSA key.

Also see smallstep/cli#1483.

Author: hslatman

token/token_test.go

@@ -4,6 +4,7 @@ import (
 	"crypto/ed25519"
 	"crypto/rand"
 	"crypto/rsa"
+	"math/big"
 	"reflect"
 	"testing"
 	"time"
@@ -95,10 +96,11 @@ func TestClaims_Sign(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	badKey, err := rsa.GenerateKey(rand.Reader, 123)
+	badKey, err := rsa.GenerateKey(rand.Reader, 1024)
 	if err != nil {
 		t.Fatal(err)
 	}
+	badKey.N = big.NewInt(10) // render key useless for signing
 
 	tests := []struct {
 		name    string