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/functional/test_framework/blocktools.py

@@ -41,6 +41,8 @@ from .script import (
 from .util import assert_equal
 from io import BytesIO
 
+MAX_BLOCK_SIGOPS = 20000
+
 # From BIP141
 WITNESS_COMMITMENT_HEADER = b"\xaa\x21\xa9\xed"