Revert "[qa] cause test_framework to mine the initial chain *now*"

Summary:
This reverts commit a95b44134c1886f32806895dc50bf7466bc8c1df. (revert of D1424)

D1424 makes tests unreproducible.

Test Plan: test_runner.py

Reviewers: deadalnix, Fabien, dagurval, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2465