test: consensus: Check that final transactions are valid #14926

There is no check that checks that final transactions are valid, i.e. the consensus rules could be changed (accidentally) with none of the tests failing.

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Coverage

^{Updated at: 2018-12-11T23:27:22.466952.}

Relevant coverage diff:

• https://drahtbot.github.io/reports/coverage/bitcoin/bitcoin/master/total.coverage/src/consensus/tx_verify.cpp.gcov.html
• https://drahtbot.github.io/reports/coverage/bitcoin/bitcoin/14926/total.coverage/src/consensus/tx_verify.cpp.gcov.html

utACK aaaa8eb1edba2a28916d5da6001d421c1b1b253b, thanks for adding consensus code test coverage!