Directly use CMutableTransaction more often in txvaidationcache_tests.cpp

Summary: As per title. This help getting Core PR12949 merged.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3330