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