We are going to deprecate `WITH_RUNTIME` directive. The main reason
behind this change is that `WITH_STDLIB` directive better describes
its meaning, specifically it will add kotlin stdlib to test's classpath.
Ideally, the type of `IrWhen` should be provided by type inference for
a consistent behavior. `USED_AS_EXPRESSION` from CFG isn't always
consistent with type inference, unfortunately.
The behavior is now aligned with `if`. The type of `when` is kept when
it *can* be an expression, instead of whether it is used or not.