tests: settings: enable ARM MPU

Remove CONFIG_ARM_MPU=n from all settings tests. This is possible due to
CONFIG_MPU_ALLOW_FLASH_WRITE=y automatically being enabled, which was not
true some time ago.

Enabling ARM MPU allows to detect stack overflows during tests runtime.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>