twister: Bring back scope selection rule using platform_allow

A change to the scope selection rules was introduced by #52715
but it doesn't comply with the description in the code. What's more,
the change introduces major issues i.e. non deterministic scope
selection, especially in a CI environment. More context at #57595

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>