twister: No skips for integration platforms in integration mode

All skips for platforms listed in the .yaml's integration_platforms
will be treated as errors in the integration mode ('-G/--integration').
This feature serves to guarantee a proper testing scope in CI as no
integration platform will be skipped silently.
Fixes #33874

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