test: Run invalid_txs.InputMissing test in feature_block (MarcoFalke)
https://github.com/bitcoin/bitcoin/pull/15102/commits/fac4e731a8e5f140dc3bbca4c9af6e23fc1cd107
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SegWit is _not_ active in our RegTest, so I just removed the conditional branch that refers to it.
Backport of Core [[https://github.com/bitcoin/bitcoin/pull/15102 | PR15102]]