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