tests: add skipped O_TMPFILE test

Looks like the FUSE protocol does support O_TMPFILE yet.

https://github.com/rfjakob/gocryptfs/issues/641

tests/defaults/overlayfs_test.go

@@ -107,3 +107,13 @@ func TestRenameExchange(t *testing.T) {
 		}
 	}
 }
+
+// Looks like the FUSE protocol does support O_TMPFILE yet
+func TestOTmpfile(t *testing.T) {
+	p := test_helpers.DefaultPlainDir + "/" + t.Name()
+	fd, err := unix.Openat(-1, p, unix.O_TMPFILE, 0600)
+	if err != nil {
+		t.Skip(err)
+	}
+	unix.Close(fd)
+}