tests: kernel: mem_protect: protection: fix test-case filter

The test is to run for boards that have memory protection
enabled; having MPU capabilities on the SoC level is not
sufficient (the user, or the board itself, might not enable
memory protection support). This commit applies that policy
to the mem_protect/protection test suite.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>