Fix multiprocessing testcase

[k-artem]

Need to force context re-creating inside testcase otherwise runtime exception raises:

    def set_start_method(self, method, force=False):
        if self._actual_context is not None and not force:
>           raise RuntimeError('context has already been set')
E           RuntimeError: context has already been set

Command for verification:
pytest -v tests/unit/runtime/test_ds_initialize.py

[k-artem]

@sfc-gh-truwase please have a look