59e387705c test: add invalid tx templates for use in functional tests (James O'Beirne)
Pull request description:
This change adds a list of `CTransaction`-generating templates which each correspond to a specific type of invalid transaction. We then use this list to test for a wider variety of invalid tx types in `p2p_invalid_tx.py` and `feature_block.py`.
Consolidating all invalid tx types will allow us to more easily cover all tx reject cases from a variety of tests without repeating ourselves. Validation logic doesn't differ much between mempool and block acceptance, but there *is* a difference and we should be sure we're testing both comprehensively.
Right now, I've only added templates covering the tx reject types listed below but if this approach seems worthwhile I will expand the list to be fully comprehensive.
```
bad-txns-in-belowout
bad-txns-inputs-duplicate
bad-txns-too-many-sigops
bad-txns-vin-empty
bad-txns-vout-empty
bad-txns-vout-negative
```
Tree-SHA512: 05407f4a953fbd7c44c08bb49bb989cefd39a2b05ea00f5b3c92197a3f05e1b302f789e33832445734220e1c333d133aba385740b77b84139b170c583471ce20
