tests: explicitly disable HW Stack Protection when needed

Some ARM platforms, now, enable HW Stack Protection by
default in the Board definition. So if some tests
need to run without stack protection, it is not
sufficient to disable TEST_HW_STACK_PROTECTION;
we need to explicitly disable HW_STACK_PROTECTION.

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