Update topkgating probability-mask test expectation (#8007)
After #7986, `tests/unit/moe/test_moe.py::TestTopkGate::test` started
failing because the expected mask changed. This was caught by the full
CI run
([log](https://github.com/deepspeedai/DeepSpeed/actions/runs/25724245746)).
This PR updates `TestTopkGate`'s `drop_policy='probs'` expected mask for
the `logits2` case to match the probability-based capacity selection
introduced by #7986.
Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
