Use MakeTransactionRef in serialize_tests.cpp
Use MakeTransactionRef in serialize_tests.cpp

Summary: This is extracted from Core PR12949 that do not apply cleanly.

Test Plan:
  make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3317

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