Fix an uninitialized const reference warning in hash_tests
Fix an uninitialized const reference warning in hash_tests

Summary:
Fixes the warning (Clang 11):
```
warning: variable 'nVersionDummy' is uninitialized when passed as a
const reference argument here [-Wuninitialized-const-reference]
        ::Unserialize(s, VARINT(nVersionDummy));
```

Test Plan:
With Clang 11:
  ninja check-bitcoin-hash_tests

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8444

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