Add attr_test.go

fuse/attr_test.go

+package fuse
+import (
+	"syscall"
+	"testing"
+)
+
+func TestFileMode(t *testing.T) {
+	sock := FileMode(syscall.S_IFSOCK)
+	if sock.IsDir() {
+		t.Error("Socket should not be directory")
+	}
+}