fix a memory leak in test_framework's key.py

Summary:
EC_KEY_set_private_key stores a *copy* of the provided bignum, thus the
private key here would be kept in memory permanently.

https://docs.huihoo.com/doxygen/openssl/1.0.1c/ec__key_8c.html

Test Plan: ran abc-replay-protection.py which uses this code; no problems occur.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2488