tests: secure_storage: update `integration_platforms`

Consistently use the nRF54L15 as the non-TF-M integration platform
in addition to `native_sim`, and the nRF9151 as the TF-M one.
In addition, add the `integration_platforms` specification to the
tests that were missing it.

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