unix: skip TestSCMCredentials SOCK_DGRAM test before 1.10

The SOCK_DGRAM version of the test was introduced for 1.10, and breaks
on older versions.

Fixes golang/go#22899

Change-Id: I97ce6f05a8a885541df823eae996cbf5d87428c2
Reviewed-on: https://go-review.googlesource.com/80275
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/creds_test.go

@@ -8,6 +8,7 @@ package unix_test
 
 import (
 	"bytes"
+	"go/build"
 	"net"
 	"os"
 	"syscall"
@@ -35,6 +36,11 @@ func TestSCMCredentials(t *testing.T) {
 	}
 
 	for _, tt := range socketTypeTests {
+		if tt.socketType == unix.SOCK_DGRAM && !atLeast1p10() {
+			t.Log("skipping DGRAM test on pre-1.10")
+			continue
+		}
+
 		fds, err := unix.Socketpair(unix.AF_LOCAL, tt.socketType, 0)
 		if err != nil {
 			t.Fatalf("Socketpair: %v", err)
@@ -134,3 +140,13 @@ func TestSCMCredentials(t *testing.T) {
 		}
 	}
 }
+
+// atLeast1p10 reports whether we are running on Go 1.10 or later.
+func atLeast1p10() bool {
+	for _, ver := range build.Default.ReleaseTags {
+		if ver == "go1.10" {
+			return true
+		}
+	}
+	return false
+}