Update policyestimator_tests to use COutPoint's accessor.

Summary:
Use nSequence field instead of changing the outpoint. This was done to generate a new TxId and never to have a valid outpoint to begin with, so it will do.

Depends on D1388 and D1385

Test Plan:

make check

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1401