tests: secure_storage: speed up `psa.its.secure_storage.custom.transform`

It would time out when run on-device due to the NVS setting backend being
incredibly slow once enough entries exist.

Also, rename the tests which use different store implementations to make
that explicit.

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/83210

.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>