fix UT in download

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 :warning:

Comparison is base (1d21697) 87.90% compared to head (95683c2) 87.90%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #149 +/- ## ========================================== - Coverage 87.90% 87.90% -0.01% ========================================== Files 79 79 Lines 3788 3786 -2 ========================================== - Hits 3330 3328 -2 Misses 458 458 ``` | [Impacted Files](https://codecov.io/gh/deepmodeling/dpgen2/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling) | Coverage Δ | | |---|---|---| | [dpgen2/utils/download\_dpgen2\_artifacts.py](https://codecov.io/gh/deepmodeling/dpgen2/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBnZW4yL3V0aWxzL2Rvd25sb2FkX2RwZ2VuMl9hcnRpZmFjdHMucHk=) | `90.72% <100.00%> (-0.10%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling)

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