fix a memory leak in test_framework's key.py
EC_KEY_set_private_key stores a *copy* of the provided bignum, thus the
private key here would be kept in memory permanently.
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