tests: protection: add riscv support

Execute tests are disabled for RISC-V because is isn't able
to set an execution restriction. From RISC-V documentation:
  "Instruction address-translation and protection are unaffected
  by the setting of MPRV"
MPRV is used to apply memory protection restriction when CPU is
running in machine mode (kernel).

Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>