Fix absurdly-high-fee failure in bip65-cltv-p2p.py

Summary:
This test is paying 49 coins in fees, which triggers absurdly-high-fee as a failure.
Adding a tx validity check helped immensely while debug this, so leaving it in seems prudent.

Test Plan: test_runner.py bip65-cltv-p2p

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2075