fix verify_test on go1.16

DSAWithSHA1 removed from crypto/x509 in go1.16, check for unsupported
error

go.mod

@@ -5,4 +5,5 @@ go 1.12
 require (
 	github.com/github/fakeca v0.1.0
 	golang.org/x/crypto v0.0.0-20190426145343-a29dc8fdc734
+	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
 )

go.sum

@@ -7,3 +7,5 @@ golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 h1:go1bK/D/BFZV2I8cIQd1NKEZ+0owSTG1fDTci4IqFcE=
+golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

verify_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/github/ietf-cms/protocol"
+	"golang.org/x/xerrors"
 )
 
 func ExampleSignedData() {
@@ -203,7 +204,6 @@ func TestVerifyChain(t *testing.T) {
 }
 
 func TestVerifyDSAWithSHA1(t *testing.T) {
-
 	// Created with the following openssl commands:
 	// openssl dsaparam -out dsakey.pem -genkey 1024
 	// openssl req -key dsakey.pem -new -x509 -days 365000 -out dsa.crt -sha1 -subj "/CN=foo.com"
@@ -272,6 +272,9 @@ F1Al5pA+giJh15T7Uu+p5O0J
 	sd.SetCertificates(pkcs7Certs)
 	_, err = sd.Verify(pkcs7VerifyOptions)
 	if err != nil {
+		if xerrors.Is(err, x509.ErrUnsupportedAlgorithm) {
+			return
+		}
 		t.Fatalf("Error verifying signing request: %v, err %v", *sd, err)
 	}
 	data, err := sd.GetData()