re-fix feature_cltv.py

Summary:
The core version of this test relied on nonpush opcodes in scriptsig
and thus got heavily rewritten in D2120 in order to comply with ABC's
consensus sigpushonly rule. This cleans up / fixes the test.

• Make it actually validate -1 CLTV before activation, like it used to.
• Change confusing name spendtx to fundtx. fundtx is always valid, as it does not execute CLTV.
• Integrate creation of the actual spending tx at the same time (DRY).
• Restore code that actually uses promiscuousmempoolflags; fix comment that inaccurately refers to promiscuousmempoolflags .
• Simplify the ending.

Test Plan: test_runner.py feature_cltv.py

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3095