Use TxId in policyestimator_tests.cpp

Summary: As per title, plus a few braces added for good measure.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1385