test: add invalid tx templates for use in functional tests

Add templates for easily constructing different kinds of invalid
transactions and use them in feature_block and p2p_invalid_tx.

test/lint/lint-python-dead-code.sh

@@ -16,4 +16,4 @@ fi
 vulture \
     --min-confidence 60 \
     --ignore-names "argtypes,connection_lost,connection_made,converter,data_received,daemon,errcheck,get_ecdh_key,get_privkey,is_compressed,is_fullyvalid,msg_generic,on_*,optionxform,restype,set_privkey" \
-    $(git ls-files -- "*.py" ":(exclude)contrib/")
+    $(git ls-files -- "*.py" ":(exclude)contrib/" ":(exclude)test/functional/data/invalid_txs.py")