twister: Change category of skip due to emulation_platforms

The value of emulation_platforms is a bool corresponding to
--emulation-only CLI option. Therefore, it should be registered
as Filters.CMD_LINE. Without it, twister raises errors when an
integration platform is descoped due to this option.

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