twister: Use proper warnings-to-errors command for sysbuild

If sysbuild is used a flag SB_CONFIG_COMPILER_WARNINGS_AS_ERRORS
has to be used in order to turn warings to errors on all images.
Align pytest tests with the change.

Fixes: #67360

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