tests: skip tests with -openssl=false on Go 1.4 and lower

Go versions 1.4 and lower lack NewGCMWithNonceSize(), which causes
a panic in the test.

internal/cryptocore/gcm_go1.4.go

@@ -10,6 +10,10 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
+const (
+	HaveModernGoGCM = false
+)
+
 // goGCMWrapper - This wrapper makes sure gocryptfs can be compiled on Go
 // versions 1.4 and lower that lack NewGCMWithNonceSize().
 // 128 bit GCM IVs will not work when using built-in Go crypto, obviously, when

internal/cryptocore/gcm_go1.5.go

@@ -7,6 +7,10 @@ import (
 	"crypto/cipher"
 )
 
+const (
+	HaveModernGoGCM = true
+)
+
 // goGCMWrapper - This wrapper makes sure gocryptfs can be compiled on Go
 // versions 1.4 and lower that lack NewGCMWithNonceSize().
 // 128 bit GCM IVs will not work when using built-in Go crypto, obviously, when

tests/example_filesystems/example_filesystems_test.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"testing"
 
+	"github.com/rfjakob/gocryptfs/internal/cryptocore"
 	"github.com/rfjakob/gocryptfs/tests/test_helpers"
 )
 
@@ -22,7 +23,12 @@ var opensslOpt string
 func TestMain(m *testing.M) {
 	// Make "testing.Verbose()" return the correct value
 	flag.Parse()
-	for _, opensslOpt = range []string{"-openssl=false", "-openssl=true"} {
+	variants := []string{"-openssl=true", "-openssl=false"}
+	if !cryptocore.HaveModernGoGCM {
+		fmt.Printf("Skipping Go GCM variant, Go installation is too old")
+		variants = variants[:1]
+	}
+	for _, opensslOpt = range variants {
 		if testing.Verbose() {
 			fmt.Printf("example_filesystems: testing with %q\n", opensslOpt)
 		}

tests/matrix/matrix_test.go

@@ -22,6 +22,7 @@ import (
 	"syscall"
 	"testing"
 
+	"github.com/rfjakob/gocryptfs/internal/cryptocore"
 	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/tests/test_helpers"
 )
@@ -34,7 +35,12 @@ var plaintextnames bool
 func TestMain(m *testing.M) {
 	// Make "testing.Verbose()" return the correct value
 	flag.Parse()
-	for _, openssl := range []bool{true, false} {
+	opensslVariants := []bool{true, false}
+	if !cryptocore.HaveModernGoGCM {
+		fmt.Printf("Skipping Go GCM variant, Go installation is too old")
+		opensslVariants = opensslVariants[:1]
+	}
+	for _, openssl := range opensslVariants {
 		for _, plaintextnames = range []bool{true, false} {
 			if testing.Verbose() {
 				fmt.Printf("matrix: testing openssl=%v plaintextnames=%v\n", openssl, plaintextnames)