test: add invalid tx templates for use in functional tests

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

Test Plan:

./test/functional/test_runner.py feature_block.py
./test/functional/test_runner.py p2p_invalid_tx.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6674