xtest: regression: add case 6021 object usage

Add case 6021 to test modifying and checking object usage of a
persistent object.

Link: https://github.com/OP-TEE/optee_os/issues/6495


Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>