tests/kernel/context: Correct dependency

An ifdef should have been also checking for native_sim,
let's correct it.

Note: By now native_sim builds with "native_posix compatibility mode"
enabled, which sets BOARD_NATIVE_POSIX to avoid breaking all this kind
of dependencies. But that will stop soon.
So let's fix the dependencies.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>