unix: add tests for *xattr functions

Based on the tests in github.com/pkg/xattr

Change-Id: I3af73ad538877da042602708248063c218260a2e
Reviewed-on: https://go-review.googlesource.com/114135
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_freebsd_test.go

@@ -295,3 +295,18 @@ func TestCapRightsSetAndClear(t *testing.T) {
 		t.Fatalf("Wrong rights set")
 	}
 }
+
+// stringsFromByteSlice converts a sequence of attributes to a []string.
+// On FreeBSD, each entry consists of a single byte containing the length
+// of the attribute name, followed by the attribute name.
+// The name is _not_ NULL-terminated.
+func stringsFromByteSlice(buf []byte) []string {
+	var result []string
+	i := 0
+	for i < len(buf) {
+		next := i + 1 + int(buf[i])
+		result = append(result, string(buf[i+1:next]))
+		i = next
+	}
+	return result
+}