native_simulator: Rework fuzzing support

The fuzzing support in the native_simulator had not got
to work yet. Let's remove it, and instead let the
test application handle it.

For this we make public the functions which initialize
the simulation and advance time, so the fuzzing tests
can use it.

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